1.1. Principal Doctrines
We must consider both the real purpose, and all the evidence of direct perception, to which we always refer the conclusions of opinion; otherwise, all will be full of doubt and confusion.
If you fight against all sensations, you will have no standard by which to judge even those of them which you say are false.
If you reject any single sensation, and fail to distinguish between the conclusion of opinion, as to the appearance awaiting confirmation, and that which is actually given by the sensation or feeling, or each intuitive apprehension of the mind, you will confound all other sensations, as well, with the same groundless opinion, so that you will reject every standard of judgment. And if among the mental images created by your opinion you affirm both that which awaits confirmation, and that which does not, you will not escape error, since you will have preserved the whole cause of doubt in every judgment between what is right and what is wrong.
If on each occasion, instead of referring your actions to the end of nature, you turn to some other, nearer, standard, when you are making a choice or an avoidance, your actions will not be consistent with your principles.
1.2. Letter to Herodotus
[Line ___] But those also who have made considerable progress in the survey of the main principles ought to bear in mind the scheme of the whole system set forth in its essentials. For we have frequent need of the general view, but not so often of the detailed exposition. Indeed it is necessary to go back on the main principles, and constantly to fix in one’s memory enough to give one the most essential comprehension of the truth. And in fact the accurate knowledge of details will be fully discovered, if the general principles in the various departments are thoroughly grasped and borne in mind; for even in the case of one fully initiated the most essential feature in all accurate knowledge is the capacity to make a rapid use of observation and mental apprehension, and this can be done if everything is summed up in elementary principles and formulae. For it is not possible for anyone to abbreviate the complete course through the whole system, if he cannot embrace in his own mind by means of short formulae all that might be set out with accuracy in detail.
[Line ___] First of all, Herodotus, we must grasp the ideas attached to words, in order that we may be able to refer to them and so to judge the inferences of opinion or problems of investigation or reflection, so that we may not either leave everything uncertain and go on explaining to infinity or use words devoid of meaning. For this purpose it is essential that the first mental image associated with each word should be regarded, and that there should be no need of explanation, if we are really to have a standard to which to refer a problem of investigation or reflection or a mental inference. And besides we must keep all our investigations in accord with our sensations, and in particular with the immediate apprehensions whether of the mind or of any one of the instruments of judgment, and likewise in accord with the feelings existing in us, in order that we may have indications whereby we may judge both the problem of sense perception and the unseen. Having made these points clear, we must now consider things imperceptible to the senses. First of all, that nothing is created out of that which does not exist: for if it were, everything would be created out of everything with no need of seeds. And again, if that which disappears were destroyed into that which did not exist, all things would have perished, since that into which they were dissolved would not exist. Furthermore, the universe always was such as it is now, and always will be the same. For there is nothing into which it changes: for outside the universe there is nothing which could come into it and bring about the change. Moreover, the universe is bodies and space: for that bodies exist, sense itself witnesses in the experience of all men, and in accordance with the evidence of sense we must of necessity judge of the imperceptible by reasoning, as I have already said. And if there were not that which we term void and place and intangible existence, bodies would have nowhere to exist and nothing through which to move, as they are seen to move. And besides these two, nothing can even be thought of either by conception or on the analogy of things conceivable such as could be grasped as whole existences and not spoken of as the accidents or properties of such existences.
[Line ___] For not one of these beliefs is contradicted by our sensations, if one looks to see in what way sensation will bring us the clear visions from external objects, and in what way again the corresponding sequences of qualities and movements. Now we must suppose too that it is when something enters us from external objects that we not only see but think of their shapes. For external objects could not make on us an impression of the nature of their own colour and shape by means of the air which lies between us and them, nor again by means of the rays or effluences of any sort which pass from us to them — nearly so well as if models, similar in color and shape, leave the objects and enter according to their respective size either into our sight or into our mind; moving along swiftly, and so by this means reproducing the image of a single continuous thing and preserving the corresponding sequence of qualities and movements from the original object as the result of their uniform contact with us, kept up by the vibration of the atoms deep in the interior of the concrete body. And every image which we obtain by an act of apprehension on the part of the mind or of the sense-organs, whether of shape or of properties, this image is the shape or the properties of the concrete object, and is produced by the constant repetition of the image or the impression it has left. Now falsehood and error always lie in the addition of opinion with regard to what is waiting to be confirmed or not contradicted, and then is not confirmed or is contradicted. For the similarity between the things which exist, which we call real and the images received as a likeness of things and produced either in sleep or through some other acts of apprehension on the part of the mind or the other instruments of judgment, could never be, unless there were some effluences of this nature actually brought into contact with our senses. And error would not exist unless another kind of movement too were produced inside ourselves, closely linked to the apprehension of images, but differing from it; and it is owing to this, supposing it is not confirmed, or is contradicted, that falsehood arises; but if it is confirmed or not contradicted, it is true. Therefore we must do our best to keep this doctrine in mind, in order that on the one hand the standards of judgment dependent on the clear visions may not be undermined, and on the other error may not be as firmly established as truth and so throw all into confusion.
[Line ___] Although we cannot see the elemental material itself, the qualities of the things we see around us derive from the basic properties of these elemental materials. Moreover, we must suppose that the atoms do not possess any of the qualities belonging to perceptible things, except shape, weight, and size, and all that necessarily goes with shape. For every quality changes; but the atoms do not change at all, since there must needs be something which remains solid and indissoluble at the dissolution of compounds, which can cause changes; not changes into the nonexistent or from the non-existent, but changes effected by the shifting of position of some particles, and by the addition or departure of others. For this reason it is essential that the bodies which shift their position should be imperishable and should not possess the nature of what changes, but parts and configuration of their own. For thus much must needs remain constant. For even in things perceptible to us which change their shape by the withdrawal of matter it is seen that shape remains to them, whereas the qualities do not remain in the changing object, in the way in which shape is left behind, but are lost from the entire body.
[Line ___] For the addition of opinion with regard to the unseen, that the moments perceptible only by thought will also contain continuity of motion, is not true in such cases; for we must remember that it is what we observe with the senses or grasp with the mind by an apprehension that is true. Nor must it either be supposed that in moments perceptible only by thought the moving body too passes to the several places to which its component atoms move (for this too is unthinkable, and in that case, when it arrives all together in a sensible period of time from any point that may be in the infinite void, it would not be taking its departure from the place from which we apprehend its motion); for the motion of the whole body will be the outward expression of its internal collisions, even though up to the limits of perception we suppose the speed of its motion not to be retarded by collision. It is of advantage to grasp this first principle as well. Next, referring always to the sensations and the feelings, for in this way you will obtain the most trustworthy ground of belief, you must consider that the soul is a body of fine particles distributed throughout the whole structure, and most resembling wind with a certain admixture of heat, and in some respects like to one of these and in some to the other.
[Line ___] Moreover, if the whole structure is dissolved, the soul is dispersed and no longer has the same powers nor performs its movements, so that it does not possess sensation either. For it is impossible to imagine it with sensation, if it is not in this organism and cannot effect these movements, when what encloses and surrounds it is no longer the same as the surroundings in which it now exists and performs these movements. Furthermore, we must clearly comprehend as well, that the incorporeal in the general acceptation of the term is applied to that which could be thought of as such as an independent existence. Now it is impossible to conceive the incorporeal as a separate existence, except the void: and the void can neither act nor be acted upon, but only provides opportunity of motion through itself to bodies. So that those who say that the soul is incorporeal are talking idly. For it would not be able to act or be acted on in any respect, if it were of this nature. But as it is, both these occurrences are clearly distinguished in respect of the soul. Now if one refers all these reasonings about the soul to the standards of feeling and sensation and remembers what was said at the outset, he will see that they are sufficiently embraced in these general formulae to enable him to work out with certainty on this basis the details of the system as well. Moreover, as regards shape and colour and size and weight and all other things that are predicated of body, as though they were concomitant properties either of all things or of things visible or recognizable through the sensation of these qualities, we must not suppose that they are either independent existences (for it is impossible to imagine that), nor that they absolutely do not exist, nor that they are some other kind of incorporeal existence accompanying body, nor that they are material parts of body: rather we should suppose that the whole body in its totality owes its own permanent existence to all these, yet not in the sense that it is composed of properties brought together to form it (as when, for instance, a larger structure is put together out of the parts which compose it, whether the first units of size or other parts smaller than itself, whatever it is), but only, as I say, that it owes its own permanent existence to all of them. All these properties have their own peculiar means of being perceived and distinguished, provided always that the aggregate body goes along with them and is never wrested from them, but in virtue of its comprehension as an aggregate of qualities acquires the predicate of body. Furthermore, there often happen to bodies and yet do not permanently accompany them accidents, of which we must suppose neither that they do not exist at all nor that they have the nature of a whole body, nor that they can be classed among unseen things nor as incorporeal. So that when according to the most general usage we employ this name, we make it clear that accidents have neither the nature of the whole, which we comprehend in its aggregate and call body, nor that of the qualities which permanently accompany it, without which a given body cannot be conceived. But as the result of certain acts of apprehension, provided the aggregate body goes along with them, they might each be given this name, but only on occasions when each one of them is seen to occur, since accidents are not permanent accompaniments. And we must not banish this clear vision from the realm of existence, because it does not possess the nature of the whole to which it is joined nor that of the permanent accompaniments, nor must we suppose that such contingencies exist independently (for this is inconceivable both with regard to them and to the permanent properties), but, just as it appears in sensation, we must think of them all as accidents occurring to bodies, and that not as permanent accompaniments, or again as having in themselves a place in the ranks of material existence; rather they are seen to be just what our actual sensation shows their proper character to be. Moreover, you must firmly grasp this point as well; we must not look for time, as we do for all other things which we look for in an object, by referring them to the general conceptions which we perceive in our own minds, but we must take the direct intuition, in accordance with which we speak of “a long time” or “a short time,” and examine it, applying our intuition to time as we do to other things. Neither must we search for expressions as likely to be better, but employ just those which are in common use about it. Nor again must we predicate of time anything else as having the same essential nature as this special perception, as some people do, but we must turn our thoughts particularly to that only with which we associate this peculiar perception and by which we measure it.
1.3. Letter to Pythocles
[Line 94, translation by ____] The wanings of the moon and its subsequent waxings ..... may be explained in all the ways in which phenomena on earth invite us to such explanations of these phases, if only one does not fall in love with the method of a single explanation (μοναχῇ τρόπος ) and groundlessly disapproves of others, without having considered what it is possible for a human being to observe and what it is not and, for this reason, desirous of observing things that cannot be observed’.
1.4. On Nature XI
[Line Ia11-19; Sedley] The sun, if we walk towards the place from which it appeared to us] to rise, directing ourselves up into the mainland zone, appears to us to set where we previously passed by, sometimes even when we have moved in all only a short distance. And this time we cannot blame it on the latitudinal movements. Why after all should you declare the measurement from here, or the one from here, or the one from here, or this one a more reliable guide of the risings and settings (of the sun)?
1.5. On Nature XI
[Line IIa1 - 21; Sedley] They cannot hope] to form a [mental] model ([ὁ]μοίωμα) and to reason out (συλλογίζεσθαι) anything about these matters. For it seems to me that when they spend their time contriving some of them (I means their [ὄρ]γανα, instruments) and fooling around with others, it is no wonder, in view not only of the enslavements brought upon them by their doctrines but also (as far as concerns the appearances of the sun) of the indeterminacies (ἀοριστείας) of risings and settings, that they cannot form an adequate mental model by means of their instruments which produce no regularity. But their instruments are ...
2.1. Book I
 For that body exists is declared by the feeling which all share alike; and unless faith in this feeling be firmly grounded at once and prevail, there will be naught to which we can make appeal about things hidden, so as to prove aught by the reasoning of the mind.
2.2. Book IV
[Line no. - Cyril Bailey] Again, if any one thinks that nothing is known, he knows not whether that can be known either, since he admits that he knows nothing. Against him then I will refrain from joining issue, who plants himself with his head in the place of his feet. And yet were I to grant that he knows this too, yet I would ask this one question; since he has never before seen any truth in things, whence does he know what is knowing, and not knowing each in turn, what thing has begotten the concept of the true and the false, what thing has proved that the doubtful differs from the certain? You will find that the concept of the true is begotten first from the senses, and that the senses cannot be gainsaid. For something must be found with a greater surety, which can of its own authority refute the false by the true. Next then, what must be held to be of greater surety than sense? Will reason, sprung from false sensation, avail to speak against the senses, when it is wholly sprung from the senses? For unless they are true, all reason too becomes false. Or will the ears be able to pass judgement on the eyes, or touch on the ears? or again will the taste in the mouth refute this touch; will the nostrils disprove it, or the eyes show it false? It is not so, I trow. For each sense has its faculty set apart, each its own power, and so it must needs be that we perceive in one way what is soft or cold or hot, and in another the diverse colours of things, and see all that goes along with colour. Likewise, the taste of the mouth has its power apart; in one way smells arise, in another sounds. And so it must needs be that one sense cannot prove another false. Nor again will they be able to pass judgement on themselves, since equal trust must at all times be placed in them.
Therefore, whatever they have perceived on each occasion, is true. And if reason is unable to unravel the cause, why those things which close at hand were square, are seen round from a distance, still it is better through lack of reasoning to be at fault in accounting for the causes of either shape, rather than to let things clear seen slip abroad from your grasp, and to assail the grounds of belief, and to pluck up the whole foundations on which life and existence rest. For not only would all reasoning fall away; life itself too would collapse straightway, unless you chose to trust the senses, and avoid headlong spots and all other things of this kind which must be shunned, and to make for what is opposite to these. Know, then, that all this is but an empty store of words, which has been drawn up and arrayed against the senses. Again, just as in a building, if the first ruler is awry, and if the square is wrong and out of the straight lines, if the level sags a whit in any place, it must needs be that the whole structure will be made faulty and crooked, all awry, bulging, leaning forwards or backwards, and out of harmony, so that some parts seem already to long to fall, or do fall, all betrayed by the first wrong measurements; even so then your reasoning of things must be awry and false, which all springs from false senses.
3. Diogenes of Oinoanda - The Inscription
[Line No. __ Martin Ferguson Smith] [Others do not] explicitly [stigmatise] natural science as unnecessary, being ashamed to acknowledge [this], but use another means of discarding it. For, when they assert that things are inapprehensible, what else are they saying than that there is no need for us to pursue natural science? After all, who will choose to seek what he can never find?
Now Aristotle and those who hold the same Peripatetic views as Aristotle say that nothing is scientifically knowable, because things are continually in flux and, on account of the rapidity of the flux, evade our apprehension. We on the other hand acknowledge their flux, but not its being so rapid that the nature of each thing [is] at no time apprehensible by sense-perception. And indeed [in no way would the upholders of] the view under discussion have been able to say (and this is just what they do [maintain] that [at one time] this is [white] and this black, while [at another time] neither this is [white nor] that black, [if] they had not had [previous] knowledge of the nature of both white and black.