1. Knowledge Is Possible - He Who Says "Nothing Can Be Known" Knows Nothing
Epicurus was strongly opposed to the radical skepticism that alleges all genuine knowledge of reality is impossible. His response began with the observation that such claims are self-refuting: the person who asserts "nothing can be known" is asserting that their own claim is without any basis, since it too falls within the class of things that supposedly cannot be known. Radical skepticism therefore collapses under its own weight when examined honestly.
More positively, Epicurus insisted that nature has given us the tools we need for reliable knowledge: the sensations, anticipations (prolepseis), and feelings that together constitute his Canon of Truth. These are not arbitrary human constructions but natural faculties present in every living creature. Sensation is our direct physical contact with external reality; anticipations (preconceptions) are the general concepts built up from repeated sensory experience that allow us to recognize and name what we encounter; and feelings of pleasure and pain are the evaluative dimension of experience that guides all choice and avoidance. These faculties do not function perfectly at all times -- eyes can be near-sighted, judgment can be rushed -- but the faculties themselves are genuine and reliable at their foundation. To claim they are worthless is to leave us with nothing: no ability to act, no ability to reason, no ability to evaluate anything. A philosophy that genuinely embraces radical skepticism cannot even recommend itself, which is why all productive philosophical work must take reliable knowledge as both possible and necessary.
The practical consequence is liberating rather than merely academic: if the senses are reliable, if preconceptions are genuine, if feelings of pleasure and pain are real guides, then the therapeutic project of Epicurean philosophy -- correcting false beliefs, removing groundless fears, and building a pleasurable life -- is genuinely achievable. Knowledge is not a luxury or an abstraction; it is the foundation of the happy life.
First of all we must grasp the ideas attached to words, so that we can have them to refer to as we proceed, and so that we may not have everything undecided for us as we try to judge things, or have words that are empty of meaning. For the primary concept corresponding to each word must be seen and needs no proof, if we are going to have something to which the point at issue or the problem or the opinion can be referred. (Bailey)
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. (Bailey)
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. (tr. Long & Sedley)
For as dogs often discover by smell the lair of a mountain-ranging wild beast, though covered over with leaves, when once they have got on the sure track, thus you in cases like this will be able by yourself alone to see one thing after another and find your way into all dark corners and draw forth the truth. (Munro)
Wherefore those who have thought that fire is the substance of things, and that the universe may consist of fire alone, are seen to fall very far from true reasoning. Heraclitus is their leader, of bright fame for his dark sayings, yet rather among the empty-headed than among the Greeks of weight, who seek after the truth. For fools laud and love all things more which they can descry hidden beneath twisted sayings, and they set up for true what can tickle the ear with a pretty sound and is tricked out with a smart ring. (Bailey)
Wondrously many other things of this sort we see, all of which would fain spoil our trust in the senses; all in vain, since the greatest part of these things deceives us on account of the opinions of the mind, which we add ourselves, so that things not seen by the senses are counted as seen. For nothing is harder than to distinguish things manifest from things uncertain, which the mind straightway adds of itself. (Bailey)
It was indeed excellently said by Epicurus that fortune only in a small degree crosses the wise man's path, and that his greatest and most important undertakings are executed in accordance with his own design and his own principles, and that no greater pleasure can be reaped from a life which is without end in time, than is reaped from this which we know to have its allotted end. He judged that the logic of your school possesses no efficacy either for the amelioration of life or for the facilitation of debate. He laid the greatest stress on natural science; when we have learned the constitution of the universe we are relieved of superstition, are emancipated from the dread of death, are not confused through ignorance of things, from which condition frightful terrors often arise; finally, our characters will also be improved when we have learned what it is that nature craves. Then again if we grasp a firm knowledge of phenomena, and uphold that canon, which almost fell from heaven into human ken, if by this means we test all our judgments, we shall never succumb to any man's eloquence and abandon our opinions. (Reid)
Moreover, unless the constitution of the world is thoroughly understood, we shall by no means be able to justify the verdicts of our senses. Further, our mental perceptions all arise from our sensations; and if these are all to be true, as the system of Epicurus proves to us, then only will cognition and perception become possible. Now those who invalidate sensations and say that perception is altogether impossible, cannot even clear the way for this very argument of theirs when they have thrust the senses aside. Moreover, when cognition and knowledge have been invalidated, every principle concerning the conduct of life and the performance of its business becomes invalidated. So from natural science we borrow courage to withstand the fear of death, and firmness to face superstitious dread, and tranquility of mind, through the removal of ignorance concerning the mysteries of nature, and self-control, which arises from the elucidation of the nature of the desires and their different classes; and, as I shewed just now, our leader again has established the canon and criterion of knowledge and thus has imparted to us a method for marking off falsehood from truth. (Reid)
Now, if someone thinks that nothing is known, one thing he doesn't know is whether that can be known, since he admits to knowing nothing. I shall therefore not bother to argue my case against this man who has himself stood with his own head in his footprints, And anyway, even allowing that he knows this, I'll still ask him: Given that he has never before seen anything true in the world, from where does he get his knowledge of what knowing and not knowing are? What created his preconception of true and false? And what proved to him that doubtful differs from certain? You will find that the preconception of true has its origin in the senses, and that the senses cannot be refuted. For something of greater reliability must be found, something possessing the intrinsic power to convict falsehoods with truths. Well, what should be considered to have greater reliability than the senses? Will reason have the power to contradict them, if it is itself the product of false sensation? For reason is in its entirety the product of the senses, so that if the senses are not true all reason becomes false as well. Or will the ears have the power to confute the eyes, and touch to confute the ears? Or again, will this sense of touch be denounced by the mouth's taste, confuted by the nose, or convicted by the eyes? That is not, in my view, the way things are. For each has its own separate capacity and its own power, thus making it necessary that sensing what is soft, cold or hot be a separate operation from sensing the various colors of things and seeing whatever properties regularly accompany colors. Likewise the mouth's taste has a separate power, the recognition of smells is separate, and separate again that of sounds. It necessarily follows that the senses cannot convict each other. Nor again will they be able to confute themselves, since all will always have to be considered of equal reliability. Hence whatever impression the senses get at any time is true. Even if reason fails to explain why things which proved square when close up seem round at a distance, it is nevertheless better, when one's reason proves inadequate, to give wrong explanations of the respective shapes, than to let the self-evident slip from one's grasp and thus to violate the primary guarantee and shake the entire foundations on which life and survival rest. For not only would all reason cave in, but life itself would instantly collapse, if you lost the confidence to trust your senses, and to avoid precipices and other such hazards while aiming towards things of the opposite kind. Hence you will find that the entire battalion of words which has been marshaled and armed against the senses is futile. Lastly, just as in a building, if the yardstick is defective at the outset, if the set square is misleading for lack of straight edges, and if the level has the slightest wobble anywhere in it, the inevitable result is that the whole house is made wrongly -- crooked, distorted, bulging backwards and forwards, misproportioned -- so much so that some parts seem already determined to cave in, and do cave in, all betrayed by false initial criteria. So too you will find that any account of the world must be distorted and false if it is based upon the falsity of the senses. (tr. Long & Sedley)
Platonism and skepticism were among his chief abominations... The philosophy of Epicurus was an immediate reaction to the skepticism of Pyrrho.