Frequently Asked Questions

1. Nothing comes from nothing and nothing goes to nothing, and therefore the universe as a whole has existed eternally, and the universe was not created at any single point in time, neither by a god nor by any other single event.
2. Everything in the universe is composed of combinations of elementary matter and void. Therefore nothing else exists - no Religious "heaven" or "hell," no spiritual or other dimension, no Platonic "ideals," no Aristotelian "essences." Nothing exists except elementary matter, void, and their combinations.
3. The amount of elementary matter and void in the universe is unlimited in extent. Therefore the universe as a whole is unlimited in extent, and the universe as a whole has no center and no edge, and the earth is not uniquely positioned at the center of the universe.
4. The elementary matter is always in motion, and therefore the universe is constantly changing, but not so fast that we are unable to grasp the truths that are relevant to us.
5. The elementary matter moves through the void at a uniform speed, but vibrates in compounds due to collisions, and therefore the change in the universe is not chaotic, but subject to laws of motion.
6. The elementary matter is capable of swerving from its path at no fixed place or time, and therefore not everything in the universe is predetermined from the beginning of time.
7. The elementary matter has varying weights, shapes, and sizes, but the number of these variations not infinite, only innumerable. Therefore the properties of elementary matter and the combinations formed by elementary matter and void are not unlimited, but limited by the properties of the elementary matter and its combinations.
8. Combinations of elementary matter and void are not by nature eternal but are created and destroyed. Therefore while the universe as a whole is eternal, the combinations of elementary matter and void, including the Earth, are not eternal.
9. To the extent that "perfect" combinations of elementary matter and void exist, such perfect things (including perfect beings) neither cause nor receive trouble, because causing and receiving trouble are characteristics only of things which are weak. Therefore to the extent that any life forms, or any combinations of elementary matter and void, have developed the capacity to live without end, they neither cause change to us nor or are they changed by any actions we may take.
10. Nature never creates only a single instance of any kind of thing. Therefore life exists throughout the universe, as the Earth cannot possibly be the only place where life exists.
11. In the Universe as a whole, every thing has its match and counterpart. This principle of uniform distribution is known as "isonomia." Therefore, for example, there are a comparable number of "immortal" beings as "mortal" beings in the universe.

References for this list may be found here.

Discussion of this FAQ entry is here.

This question rests on a misunderstanding of what Epicurean physics is for.

The Common Version of the Question

The question usually takes one of these forms: "Epicurus talked about atoms, but we now know about subatomic particles, quantum mechanics, mass-energy equivalence, and the like. Since Epicurus was wrong about the technical details, shouldn't we just treat his physics as a charming historical curiosity and focus on the lifestyle advice? Why does it matter whether Epicurus was a 'reductionist' or not? Why should I care about ancient atomic theory at all?"

The short answer is: you should care not because the technical details of ancient atomic theory are sacred or unchangeable, but because Epicurean physics was designed to accomplish something that remains as urgently necessary today as it was in the ancient world — and that something has nothing to do with the specific size or weight of atoms.

What Epicurean Physics Was Actually Built To Do

Epicurus constructed his physics — atoms, void, the swerve, the emergence of higher-level qualities from atomic combinations — to perform a very specific philosophical function. He built it to eliminate two sources of devastating human misery that his culture (and ours) constantly reinforces:

First, the fear that supernatural gods are observing human life, intervening in it, and punishing people either in this life or after death. This fear, Epicurus argued, was the root of enormous unnecessary suffering — it produced anxiety, superstition, and an inability to live fully in the world we actually inhabit.

Second, the fear — or the resignation — that comes from believing the universe is governed by Fate or Necessity, that human choices are determined in advance, and that individuals have no real agency over their own lives.

Epicurean physics was built to demonstrate, through careful reasoning from observable nature, that neither of these fears has any foundation in reality. Nothing comes from nothing; nothing goes to nothing; the universe operates by natural processes at every level — not by divine command, not by fate imposed from outside. If you understand that and believe it on the basis of evidence rather than authority, you are free to live on your own terms.

This is why Epicurus himself said, in the opening of the Letter to Herodotus, that even a brief summary of his physics was worth having — because the ability to refer to it confidently at any moment was essential to living without unnecessary fear.

The Technical Details Update; The Framework Does Not

Now — does it matter that we now know atoms are not the ultimate indivisible units of matter? That what Epicurus called "atoms" we would today call something more like fundamental particles or quanta of mass-energy? That modern cosmology has added dimensions to our picture of the universe that Epicurus could not have imagined?

Of course it matters, in the sense that an honest commitment to observation and evidence — which is the very core of the Epicurean approach — demands that we incorporate new knowledge rather than ignore it. Epicurus himself would insist on this. The philosopher who held that all sensations are true (in the sense of honestly reported data about the world) is never going to turn his back on reliable, reproducible scientific observations.

But here is the key distinction: the specific technical vocabulary of ancient atomic theory (which was, after all, the best available working model for a materialist understanding of nature in the 4th century BC) can be updated without abandoning the framework. What remains constant is the method and the core conclusions:

• Nature is self-sufficient and self-explanatory. It does not require supernatural governance.
• At some level of fundamental analysis, there are irreducible constituents of reality that give the universe its stability and continuity. We observe this stability every day; we don't need to speculate about miracles.
• Nothing we observe in nature suggests that supernatural gods are intervening in the physical world or in human affairs.
• Nothing we observe in nature requires us to accept strict determinism — the universe has genuine openness that makes human agency real.

Modern physics has not overturned any of these conclusions. If anything, it has deepened them.

The World Around You Is Fully Real — Not "Merely" Atoms, and Not a Shadow of Some Higher Realm

This is perhaps the most important practical consequence of Epicurean physics for how you actually live, and it deserves to be stated plainly.

Epicurus was not a strict reductionist. He did not teach that the only reality is atoms moving through void, and that everything we experience — color, warmth, pleasure, pain, friendship, beauty — is therefore somehow an illusion or a second-class substitute for the "real" reality underneath. As David Sedley argued persuasively in his 1988 paper "Epicurean Anti-Reductionism," and as Lucretius makes clear throughout On the Nature of Things, the emergent qualities of compound things are genuinely real features of the world, not mere appearances to be explained away. The color of a flower, the warmth of a fire, the pleasure of a meal shared with a friend — these are not illusions projected onto an underlying atomic reality. They are real. They are the world.

This matters enormously, and it places Epicurus in direct opposition to attacks on the reality of lived life coming from two directions at once:

From the direction of strict scientific reductionism, the attack goes: "Your pleasures and pains are just brain chemistry. Your love for your friends is just neurons firing. There is nothing 'really real' about your experience that needs to be taken seriously as a guide to life — the only genuine reality is physics." Epicurus rejects this. Lived experience is not a by-product to be explained away; it is the primary datum. Pleasure and pain are real. The things you love are real. Your life is not a shadow of something more fundamental — it is the fundamental thing at the level at which you exist.

From the direction of religion and supernaturalism, the attack goes: "The world you see around you is transient, material, and ultimately unimportant. True reality lies beyond — in a god, a heaven, a realm of eternal forms, or a life after death. Don't be fooled by the pleasures of this world; they are mere illusions compared to what awaits beyond." Epicurus rejects this equally firmly. There is no higher realm. The world you inhabit is the real world, fully and completely. The people around you are real. Your life as you are living it right now is not a rehearsal or a shadow — it is the thing itself.

Both attacks, though they come from opposite directions, share a common strategy: they devalue the world you actually inhabit in order to redirect your attention and your loyalties somewhere else — to physics equations, or to gods and afterlives. Epicurean physics, properly understood, is a defense of the full reality of the life in front of you. This is why physics and ethics are inseparable in Epicurean thought, and why Epicurus made his students study physics first.

Why the Reductionism Debate Matters to Everyday Life

This brings us to a point that the ongoing scholarly discussion of Epicurean physics — including Sedley's 1988 paper on "Epicurean Anti-Reductionism" — makes surprisingly concrete.

Sedley's argument (and Lucretius' text strongly supports it) is that Epicurus did not teach that everything real can be reduced without remainder to atomic descriptions alone. While everything is composed of atoms and void at the ontological level, the emergent qualities of compound things — color, heat, life, pleasure, pain, psychological experience — are genuine features of the world, not illusions to be explained away. They must be understood at their own level.

Why does this matter to how you live? Because the most common dismissal of Epicurean ethics in our own time sounds something like this: "Pleasure and pain are just brain chemistry. Your feelings are ultimately just atoms bumping together. There's nothing 'really real' about your subjective experience that demands to be taken seriously as a guide to life." This is, ironically, the kind of strict eliminative reductionism that Epicurus himself rejected. His physics was not built to undermine the reality of lived experience — it was built to ground it. Pleasure and pain are real. They are the data of living. They cannot be argued away into physics, even if physics ultimately underlies them.

So the atoms debate is not merely academic. Getting it right protects the ethical conclusions of the philosophy.

Engaging Confidently with Modern Science

For all of the above reasons, we encourage participants here to engage confidently with modern science rather than to retreat from the comparison. When someone says "Epicurus was wrong about atoms, so his philosophy fails," the answer is not defensive embarrassment but something like this:

Epicurus' commitment was to the method of observation-based reasoning and to the framework conclusion that nature is self-sufficient. The specific working model of his day was the best available — and it was built explicitly on reasoning that remains valid: nothing from nothing, nothing to nothing, no supernatural causation ever observed. The details of particle physics, quantum mechanics, or cosmology do not overturn those conclusions. They extend and refine them. An Epicurean engaging seriously with modern physics should feel vindicated, not threatened.

The universe does not require gods to explain it. It does not require fate to organize it. It gives us, at the emergent level of human life, genuine pleasure and genuine pain as real guides to how to live. That remains the core of what Epicurean physics was built to establish — and on all of those points, the last 2,000 years of science have done nothing but confirm the framework.

For discussion of this FAQ, see the forum here: [Article - David Sedley - 1988 - "Epicurean Anti-Reductionism"]

Older version of this answer:

Here's an example of this question: "What is the Epicurean physics view of energy? Instead of "matter and void", shouldn't it be "mass-energy and void"? How strongly do we hold to the idea that atoms are indestructible and immutable? What would Epicurus think if he knew about matter-antimatter annihilation?"

But first, the general way this question is often asked comes down to something like this: "Don't we now know about subatomic particles and other phenomena smaller than atoms, and since Epicurus said atoms were indivisible then Epicurus was wrong, his philosophy fails, he can't be trusted, so shouldn't we just muse about how interesting it is to consider pleasure the goal of life, and how cute he was to talk about pleasure as "absence of pain?" Shouldn't we just discuss Epicurus for an hour while we have a beer and eat some exotic food and after that go back to studying Plato and the Stoics?"

And the general answer to that question is "No."

That's because the Epicurean view of nature was built on an approach to knowledge that is first and foremost geared toward adapting to and incorporating all observations that can reliably judged to be correct. The philosopher known for the viewpoint that "all sensations are true" (in the sense of honestly reported) is never going to ignore new observations in physics which are repeatedly and reliably observed. Read Lucretius and you will see the most detailed presentation of Epicurean physics left to us, and you will see that the physics is built on a step by step series of observations that remain persuasive today. No doubt some will want to argue about this, but the general starting points that (1) nothing ever truly comes from nothing, and (2) nothing ever truly goes to nothing remain persuasive today. Even more certainly, neither of those phenomena are ever observed to occur at the whim or will of any supernatural god. All of the rest of Epicurean physics are derivative conclusions intended to produce a working model of how this fundamental observation is most likely to be "explainable" given the knowledge that we have. And one part of Epicurus' working model was that at some point in nature we arrive at an irreducible limit where things can no longer be divided further, and that this are of limit is where nature gets its stability and reliability and continuity that we see around us every day, which exists by nature and not because a supernatural god is watching it and willing it into existence.

It is therefore fundamental to observe that if new instrumentation gives us the ability to prove to our satisfaction the existence of "mass-energy" or anything else then that would be incorporated into the overall consistent world-view.

One also has to consider that the use of words varies between languages and over the centuries. When Epicurus was referring to "atoms" the Greek translates most generally into "things that are indivisible. What we refer to today as atoms made up of subatomic particles would easily be incorporated into Epicurean physics by observing that what Epicurus was really saying was that at SOME point you come to a level where existence is indivisible.

Various philosophers of Epicurus day and before had asserted that matter was theoretically infinitely divisible, and they carried those observations to ridiculous conclusions such as that movement is impossible. Epicurean physics is largely devoted to philosophical approach that prioritizes observation and practical experience over abstract theory, meaning for just one example that when we observe that motion is going on everywhere around us, we do not accept speculative abstractions which assert that motion does not exist. The entire issue of supernatural gods and supernatural realms is essentially on this same level - it is the assertion of the existence of profoundly important things that (if true) would lead to an entirely different set of ethics and moral values than is otherwise the case.

You could apply the same analysis to "mass-energy" or "matter-antimatter" or astrophysics or any other science. Epicurus was committed to living in the real world that we experience as human beings, and if a speculative theory led in his own time to a conclusion that contradicts human experience, as we experience it through our human faculties, then such theories are slated for rejection. That doesn't mean that we are ever wedded to the details of any one theory of physics, and in fact sometimes we have to "wait" in choosing between theories that seem consistent with the facts but for which we don't have enough facts to be sure which is correct.

This attitude showed Epicurus' commitment to using a reasonable approach to what we can be confident of and what we cannot, because Epicurus knew that if we don't consciously separate the things about which we are confident from those that we aren't then there is no essential difference between us and a fanatical religionist, because we would be accepting things on "Faith" rather than on rigorous commitment to following the evidence.

That's a start at the general answer but there's a lot more to say.

Thread for discussion of this FAQ is here.

Short Answer: That depends on your definition of the word “atheist.” The American Heritage Dictionary defines “atheism” as “Disbelief in or denial of the existence of God or gods.” Under this definition, which does not specify that gods are “all-powerful” or that gods created the universe, Epicurus was not an atheist. Epicurus held there to be a race of perfect, immortal gods living in distant parts of the universe who neither created the universe, control it, or have any concern for the happenings on Earth.

From the opening of Epicurus’ Letter to Menoeceus: “First believe that God is a living being immortal and blessed, according to the notion of a god indicated by the common sense of mankind; and so believing, you shall not affirm of him anything that is foreign to his immortality or that is repugnant to his blessedness. Believe about him whatever may uphold both his blessedness and his immortality. For there are gods, and the knowledge of them is manifest; but they are not such as the multitude believe, seeing that men do not steadfastly maintain the notions they form respecting them. Not the man who denies the gods worshiped by the multitude, but he who affirms of the gods what the multitude believes about them is truly impious. For the utterances of the multitude about the gods are not true preconceptions but false assumptions; hence it is that the greatest evils happen to the wicked and the greatest blessings happen to the good from the hand of the gods, seeing that they are always favorable to their own good qualities and take pleasure in men like themselves, but reject as alien whatever is not of their kind.”

The answer is different if your definition of “atheist” requires that gods be all-powerful or responsible for creation and direction of the universe – in other words that god is a “supreme being.” For example, Dictionary.com defines “atheist” as: “a person who denies or disbelieves the existence of a supreme being or beings.” By this definition, Epicurus does qualify as an atheist, as all Epicurean texts refer to gods as “a part” of Nature, and not as “supreme above” or “superior to” or “creator of” Nature itself.

From Norman DeWitt's "Epicurus and His Philosophy":

ISONOMY AND THE GODS

In spite of a supercilious opinion to the contrary, Epicurus was not a muddled thinker but a very systematic one. He enunciated his Twelve Elementary Principles and adhered to them closely. Two of these, the fifth and sixth, asserted the infinity of the universe in respect of matter and space. To this idea of infinity he ascribed fundamental importance. He exhorted the young Pythocles to study it as one of those master principles which would render easy the recognition of causation in details.68 Cicero must have been recalling some similar exhortation when he wrote: "But of the very greatest importance is the significance of infinity and in the highest degree deserving of intense and diligent contemplation." He was quoting Epicurus.

It was from this principle that Epicurus deduced his chief theoretical confirmation of belief in the existence of gods. It was from this that he arrived at knowledge of their number and by secondary deduction at knowledge of their abode. He so interpreted the significance of infinity as to extend it from matter and space to the sphere of values, that is, to perfection and imperfection. In brief, if the universe were thought to be imperfect throughout its infinite extent, it could no longer be called infinite. This necessity of thought impelled him to promulgate a subsidiary principle, which he called isonomia, a sort of cosmic justice, according to which the imperfection in particular parts of the universe is offset by the perfection of the whole. Cicero rendered it aequabilis tributio, "equitable apportionment." The mistake of rendering it as "equilibrium" must be avoided.

The term isonomia itself, which may be anglicized as isonomy, deserves a note. That it is lacking in extant Epicurean texts, all of them elementary, and is transmitted only by Cicero is evidence of its belonging to higher doctrine and advanced studies. Epicurus switched its meaning slightly, as he did that of the word prolepsis. To the Greeks it signified equality of all before the law, a boast of Athenians in particular. It was a mate to eunomia, government by law, as opposed to barbaric despotism, a boast of Greeks in general. That Epicurus thought to make capital of this happy connotation may be considered certain. He was vindicating for Nature a sort of justice, the bad being overbalanced by the good. It is also possible that he was remotely influenced by the teachings of Zoroaster, well known in his day through the conquests of Alexander, according to whom good and evil, as represented by Ormazd and Ahriman, battled for the upper hand in mundane affairs.

Whatever may be the facts concerning this influence, Epicurus discovered a reasonable way of allowing for the triumph of good in the universe, which seemed impossible under atomic materialism. Thus in his system of thought isonomy plays a part comparable to that of teleology with Plato and Aristotle. Teleology was inferred from the evidences of design, and design presumes agencies of benevolence, whether natural or divine. Epicurus was bound to reject design because the world seemed filled with imperfections, which he listed, but by extending the doctrine of infinity to apply to values he was able, however curiously, to discover room for perfection along with imperfection.

That he employed isonomy as theoretical proof of the existence of gods is well documented. For example, Lactantius, who may have been an Epicurean before his conversion to Christianity, quotes Epicurus as arguing "that the divine exists because there is bound to be something surpassing, superlative and blessed." The necessity here appealed to is a necessity of thought, which becomes a necessity of existence. The existence of the imperfect in an infinite universe demands belief in the existence of the perfect. Cicero employs very similar language: "It is his doctrine that there are gods, because there is bound to be some surpassing being than which nothing is better." Like the statement of Lactantius, this recognizes a necessity of existence arising from a necessity of thought; the order of Nature cannot be imperfect throughout its whole extent; it is bound to culminate in something superior, that is, in gods.

It is possible to attain more precision in the exposition. Cicero, though brutally brief, exhibits some precision of statement. The infinity of the universe, as usual, serves as a major premise. This being assumed, Cicero declares: "The nature of the universe must be such that all similars correspond to all similars." One class of similars is obviously taken to be human beings, all belonging to the same grade of existence in the order of Nature. As Philodemus expresses it in a book about logic, entitled On Evidences, "It is impossible to think of Epicurus as man and Metrodorus as non-man." Another class of similars is the gods. This being understood, the truth of Cicero's next statement follows logically: "If it be granted that the number of mortals is such and such, the number of immortals is not less." 75 This reasoning calls for no exegesis, but two points are worthy of mention: first, Cicero is not precise in calling the gods immortals; according to strict doctrine they are not deathless, only incorruptible of body; the second point is that Epicurus is more polytheistic in belief than his own countrymen.

The next item, however, calls for close scrutiny. Just as human beings constitute one set of similars and the gods another, so the forces that preserve constitute one set and the forces that destroy constitute another.

At this point a sign of warning is to be raised. There is also another pair of forces that are opposed to each other, those that create and those that destroy.76 The difference is that the latter operate in each of the innumerable worlds, while the former hold sway in the universe at large. For example, in a world such as our own, which is one of many, the forces of creation have the upper hand during its youthful vigor. At long last, however, the forces of destruction gradually gain the superiority and eventually the world is dissolved into its elements.

In the universe at large, on the contrary, the situation is different and the forces opposed to each other are not those that destroy and those that create but those that destroy and those that preserve. Moreover, a new aspect of infinity is invoked, the infinity of time. The universe is eternal and unchanging. Matter can neither be created nor destroyed. The sum of things is always the same, as Lucretius says. This truth is contained in the first two of the Twelve Elementary Principles. In combination they are made to read: "The universe has always been the same as it now is and always will be the same." This can be true only on the principle that the forces that preserve are at all times superior to the forces that destroy.

It follows that Cicero was writing strictly by the book when he made his spokesman draw the following conclusion from the doctrine of isonomy: "And if the forces that destroy are innumerable, the forces that preserve must by the same token be infinite." This doctrine, it is essential to repeat, holds only for the universe at large. It is not applicable to the individual world and it does not mean that the prevalence of elephants in India is balanced by the prevalence of wolves in Russia. Isonomy does not mean "equal distribution" but "equitable apportionment." It does not denote balance or equilibrium. No two sets of similar forces are in balance; in the individual world the forces of destruction always prevail at last, and in the universe at large the forces of preservation prevail at all times.

By this time three aspects of the principles of isonomy have been brought forward: first, that in an infinite universe perfection is bound to exist as well as imperfection; that is, "that there must be some surpassing being, than which nothing is better"; second, that the number of these beings, the gods, cannot be less than the number of mortals; and third, that in the universe at large the forces of preservation always prevail over the forces of destruction.

All three of these are direct inferences from the infinity and eternity of the universe. There remains to be drawn an indirect inference of primary importance. Since in the individual worlds the forces of destruction always prevail in the end, it follows that the incorruptible gods can have their dwelling place only outside of the individual worlds, that is, in the free spaces between the worlds, the so-called intermundia, where the forces of preservation are always superior. There is more to be said on this topic in the section that follows.

Discussion of this answer is here.

Short Answer: Epicurus held that the elements from which the universe is composed are eternal. Although the atoms are constantly in motion and changing positions, such that the things we see now are not permanent, the elements themselves were never created at any point in time by any god or by any other means:

Letter to Herodotus: “To begin with, nothing comes into being out of what is non-existent. For in that case anything would have arisen out of anything, standing as it would in no need of its proper germs. And if that which disappears had been destroyed and become non-existent, everything would have perished, that into which the things were dissolved being non-existent. Moreover, the sum total of things was always such as it is now, and such it will ever remain. For there is nothing into which it can change. For outside the sum of things there is nothing which could enter into it and bring about the change.”

Discussion of this question is here.

Short Answer: Epicurus definitely held that there is no such thing as an eternal or supernatural soul, because nothing is eternal in Epicurean physics other than matter and void, and nothing is supernatural - period. On the other hand, Epicurus devoted considerable attention to explaining the phenomena that is generally called "soul" or "spirit" as referenced in the cites below.

Long Answer:

In his letter to Herodotus, Epicurus wrote that the “soul” is composed of a particularly fine type of atoms of a unique type which themselves have no sensation apart from the body, but which carry the potentiality of sentience in combination with the body. Neither these particles, nor any other particles, can combine except according to the laws of Nature: “Next, keeping in view our perceptions and feelings (for so shall we have the surest grounds for belief), we must recognize generally that the soul is a corporeal thing, composed of fine particles, dispersed all over the frame, most nearly resembling wind with an admixture of heat, in some respects like wind, in others like heat. But, again, there is the third part which exceeds the other two in the fineness of its particles and thereby keeps in closer touch with the rest of the frame. And this is shown by the mental faculties and feelings, by the ease with which the mind moves, and by thoughts, and by all those things the loss of which causes death. Further, we must keep in mind that soul has the greatest share in causing sensation. Still, it would not have had sensation, had it not been somehow confined within the rest of the frame. But the rest of the frame, though it provides this indispensable conditions for the soul, itself also has a share, derived from the soul, of the said quality; and yet does not possess all the qualities of soul. Hence on the departure of the soul it loses sentience. For it had not this power in itself; but something else, congenital with the body, supplied it to body: which other thing, through the potentiality actualized in it by means of motion, at once acquired for itself a quality of sentience, and, in virtue of the neighborhood and interconnection between them, imparted it (as I said) to the body also. Hence, so long as the soul is in the body, it never loses sentience through the removal of some other part. The containing sheaths may be dislocated in whole or in part, and portions of the soul may thereby be lost; yet in spite of this the soul, if it manage to survive, will have sentience. But the rest of the frame, whether the whole of it survives or only a part, no longer has sensation, when once those atoms have departed, which, however few in number, are required to constitute the nature of soul. Moreover, when the whole frame is broken up, the soul is scattered and has no longer the same powers as before, nor the same notions; hence it does not possess sentience either. For we cannot think of it as sentient, except it be in this composite whole and moving with these movements; nor can we so think of it when the sheaths which enclose and surround it are not the same as those in which the soul is now located and in which it performs these movements. There is the further point to be considered, what the incorporeal can be, if, I mean, according to current usage the term is applied to what can be conceived as self-existent. But it is impossible to conceive anything that is incorporeal as self-existent except empty space. And empty space cannot itself either act or be acted upon, but simply allows body to move through it. Hence those who call soul incorporeal speak foolishly. For if it were so, it could neither act nor be acted upon. But, as it is, both these properties, you see, plainly belong to soul. If, then, we bring all these arguments concerning soul to the criterion of our feelings and perceptions, and if we keep in mind the proposition stated at the outset, we shall see that the subject has been adequately comprehended in outline: which will enable us to determine the details with accuracy and confidence.

We have a further explanation of these matters in Lucretius. In Book III, Lucretius wrote (translation by HAJ Munro):“Therefore, again and again I say, you are to know that the nature of the mind and the soul has been formed of exceedingly minute seeds, since at its departure it takes away none of the weight. We are not however to suppose that this nature is single. For a certain subtle spirit mixed with heat quits men at death, and then the heat draws air along with it; there being no heat which has not air too mixed with it: for since its nature is rare, many first beginnings of air must move about through it. Thus the nature of the mind is proved to be threefold; and yet these things all together are not sufficient to produce sense; since the fact of the case does not admit that any of these can produce sense-giving motions and the thoughts which a man turns over in mind. Thus some fourth nature too must be added to these: it is altogether without name; than it nothing exists more nimble or more fine, or of smaller or smoother elements: it first transmits the sense-giving motions through the frame; for it is first stirred, made up as it is of small particles; next the heat and the unseen force of the spirit receive the motions, then the air; then all things are set in action, the blood is stirred, every part of the flesh is filled with sensation; last of all the feeling is transmitted to the bones and marrow, whether it be one of pleasure or an opposite excitement. No pain however can lightly pierce thus far nor any sharp malady make its way in, without all things being so thoroughly disordered that no room is left for life and the parts of the soul fly abroad through all the pores of the body. But commonly a stop is put to these motions on the surface as it were of the body: for this reason we are able to retain life. Now though I would fain explain in what way these are mixed up together, by what means united, when they exert their powers, the poverty of my native speech deters me sorely against my will: yet will I touch upon them and in summary fashion to the best of my ability: the first-beginnings by their mutual motions are interlaced in such a way that, none of them can be separated by itself, nor can the function of any go on divided from the rest by any interval; but they are so to say the several powers of one body.

In Book II Lucretius had previously written:

“Wherefore the bodies of the first-beginnings in time gone by moved in the same way in which now they move, and will ever hereafter be borne along in like manner , and the things which have been wont to be begotten will be begotten after the same law and will be and will grow and will wax in strength so far as is given to each by the decrees of nature. And yet we are not to suppose that all things can be joined together in all ways ; for then you would see prodigies produced on all hands, forms springing up half man half beast and sometimes tall boughs sprouting from the living body, and many limbs of land-creatures joined with those of sea-animals, nature too throughout the all-bearing lands feeding chimeras which breathed flames from noisome mouth. It is plain however that nothing of the sort is done, since we see that all things produced from fixed seeds and a fixed mother can in growing preserve the marks of their kind. This you are to know must take place after a fixed law . …. To come to another point, whatever things we perceive to have sense, you must yet admit all composed of senseless first-beginnings: manifest tokens which are open to all to apprehend, so far from refuting or contradicting this, do rather themselves take us by the hand and constrain us to believe that, as I say, living things are begotten from senseless things. …Therefore nature changes all foods into living bodies and engenders out of them all the senses of living creatures, much in the same way as she dissolves dry woods into flames and converts all things into fires. Now do you see that it is of great moment in what sort of arrangement the first-beginnings of things are severally placed and with what others they are mixed up, when they impart and receive motions? Then again what is that which strikes your mind, affects that mind and constrains it to give utterance to many different thoughts, to save you from believing that the sensible is begotten out of senseless things? Sure enough it is because stones and wood and earth however mixed together are yet unable to produce vital sense.”

Discussion of this question is here.

In the letter to Pythocles, Epicurus specifically makes clear before he starts discussing astronomy that: "But this is not the case with celestial phenomena: these at any rate admit of manifold causes for their occurrence and manifold accounts, none of them contradictory of sensation, of their nature. For in the study of nature we must not conform to empty assumptions and arbitrary laws, but follow the promptings of the facts; for our life has no need now of unreason and false opinion; our one need is untroubled existence. All things go on uninterruptedly, if all be explained by the method of plurality of c__auses in conformity with the facts, so soon as we duly understand what may be plausibly alleged respecting them. But when we pick and choose__among them, rejecting one equally consistent with the phenomena, we clearly fall away from the study of nature altogether and tumble into myth. Some phenomena within our experience afford evidence by which we may interpret what goes on in the heavens. We see bow the former really take place, but not how the celestial phenomena take place, for theiroccurrence may possibly be due to a variety of causes. However, we must observe each fact as presented, and further separate from it all the facts presented along with it, the occurrence of which from various causes is not contradicted by facts within our experience."

Then when he addresses the size of the sun, he says "The size of the sun and the remaining stars relatively to us is just as great as it appears. But in itself and actually it maybe a little larger or a little smaller, or precisely as great as it is seen to be. For so too fires of which we have experience are seen by sense when we see them at a distance. And every objection brought against this part of the theory will easily be met by anyone who attends to plain facts, as I show in my work On Nature. "

Now his reason for this conclusion is clear from this — he says that on earth, things that give off light do not appear to recede in the distance as much as those things that don't. So applying that rule here, there's no reason to think that the sun is a huge distance away, any further than the moon, so no reason to think it is huge in size. Of course a major reason he leaned toward this conclusion is that he was battling the platonists, who said they were gods, and who were trying to reduce nature down to a series of calculations. He chose incorrectly, but he was motivated by good reasons. I believe much the same explanation goes to the earth as well, which I gather they did think was round, but that since everything falls down you would fall off thebottom if you were on the other side. So there were good solid reasons the Epicureans chose the positions they did, and definitely not go along just because Epicurus said so.

Also (Credit to Joshua for this):

It's important to consider the whole proposition. Epicurus thought that the sun was;

1. Wholly material
2. In constant but not uniform motion
3. In a centerless cosmos
4. Governed by the same laws as things on Earth
5. Arose out of matter, and has a finite period of existence
6. But its matter will recombine into other things
7. About as big as it seems.

Compare Aristotle's sun;

1. Made of aether, an element that didn't exist on earth
2. In constant and uniform motion (because aetherial)
3. Set in motion by unmoved mover (god)
4. Orbiting a stationary earth that was the center of everything
5. Governed by different laws than Earth (the laws of the aether)
6. Exists in perpetuity (because aetherial)
7. Size uncertain (not mentioned, as far as I can tell)

Discussion of this issue is here.

Vatican Saying 49. (PD 12) It is impossible for someone to dispel his fears about the most important matters if he doesn’t know the nature of the universe but still gives some credence to myths. So without the study of nature there is no enjoyment of pure pleasure.

Vatican Saying 72. (PD 13) There is no advantage to obtaining protection from other men so long as we are alarmed by events above or below the earth or in general by whatever happens in the boundless universe.

No.

1. Even if there was a "big bang" in our part of the universe, and indeed that would seem likely under Epicurean physics, what Epicurus was talking about as eternal is the "universe as a whole" - not any part of it.

2. There are scientists on both sides of the question of whether the universe originated in a big bang. For scientists who reject a "universal big bang" approach, including "tired light" explanations of contrary arguments, see references here.