The major source for Epicurean doctrine is Diogenes Laërtius, "Lives and Opinions of Eminent Philosophers" Book X - Epicurus Biography.
The Principal Doctrines of Epicurus is a collection of brief sayings, called "Kuriai Doxai".
1) A blessed and imperishable being neither has trouble itself nor does it cause trouble for anyone else; therefore, it does not experience feelings of anger or indebtedness, for such feelings signify weakness.
2) Death is nothing to us, because a body that has been dispersed into elements experiences no sensations, and the absence of sensation is nothing to us.
3) Pleasure reaches its maximum limit at the removal of all sources of pain. When such pleasure is present, for as long as it lasts, there is no cause of physical nor mental pain present – nor of both together.
4) Continuous physical pain does not last long. Instead, extreme pain lasts only a very short time, and even less-extreme pain does not last for many days at once. Even protracted diseases allow periods of physical comfort that exceed feelings of pain.
5) It is impossible to live pleasantly without living wisely and honorably and justly, and it is impossible to live wisely and honorably and justly without living pleasantly. Whenever any one of these is lacking (when, for instance, one is not able to live wisely, though he lives honorably and justly) it is impossible for him to live a pleasant life.
6) That natural benefit of kingship and high office is (and only is) the degree to which they provide security from other men.
7) Some seek fame and status, thinking that they could thereby protect themselves against other men. If their lives really are secure, then they have attained a natural good; if, however, they're insecure, they still lack what they originally sought by natural instinct.
8) No pleasure is a bad thing in itself, but some pleasures are only obtainable at the cost of excessive troubles.
9) If every pleasure could be prolonged to endure in both body or mind, pleasures would never differ from one another.
10) If the things which debauched men find pleasurable put an end to all fears (such as concerns about the heavenly bodies, death, and pain) and if they revealed how we ought to limit our desires, we would have no reason to reproach them, for they would be fulfilled with pleasures from every source while experiencing no pain, neither in mind nor body, which is the chief evil of life.
11) If we were never troubled by how phenomena in the sky or death might concern us, or by our failures to grasp the limits of pains and desires, we would have no need to study nature.
12) One cannot rid himself of his primal fears if he does not understand the nature of the universe but instead suspects the truth of some mythical story. So without the study of nature, there can be no enjoyment of pure pleasure.
13) One gains nothing by securing protection from other men if he still has apprehensions about things above and beneath the earth and throughout the infinite universe.
14) Supreme power and great wealth may, to some degree, protect us from other men; but security in general depends upon peace of mind and social detachment.
15) Natural wealth is both limited and easily obtained, but vanity is insatiable.
16) Chance has little effect upon the wise man, for his greatest and highest interests are directed by reason throughout the course of life.
17) The just man is the freest of anyone from anxiety; but the unjust man is perpetually haunted by it.
18) When pain arising from need has been removed, bodily pleasure cannot increase – it merely varies. But the limit of mental pleasure is reached after we reflect upon these bodily pleasures and the related mental distress prior to fulfillment.
19) Infinite and finite time afford equal pleasure, if one measures its limits by reason. 20) Bodily pleasure seems unlimited, and to provide it would require unlimited time. But the mind, recognizing the limits of the body, and dismissing apprehensions about eternity, furnishes a complete and optimal life, so we no longer have any need of unlimited time. Nevertheless, the mind does not shun pleasure; moreover, when the end of life approaches, it does not feel remorse, as if it fell short in any way from living the best life possible.
21) He who understands the limits of life knows that things which remove pain arising from need are easy to obtain, and furnish a complete and optimal life. Thus he no longer needs things that are troublesome to attain.
22) We must consider the ultimate goal to be real, and reconcile our opinions with sensory experience; otherwise, life will be full of confusion and disturbance.
23) If you argue against all your sensations, you will then have no criterion to declare any of them false.
24) If you arbitrarily reject any one sensory experience and fail to differentiate between an opinion awaiting confirmation and what is already perceived by the senses, feelings, and every intuitive faculty of mind, you will impute trouble to all other sensory experiences, thereby rejecting every criterion. And if you concurrently affirm what awaits confirmation as well as actual sensory experience, you will still blunder, because you will foster equal reasons to doubt the truth and falsehood of everything.
25) If you do not reconcile your behavior with the goal of nature, but instead use some other criterion in matters of choice and avoidance, then there will be a conflict between theory and practice.
26) All desires which create no pain when unfulfilled are not necessary; such desires may easily be dispelled when they are seen as difficult to fulfill or likely to produce harm.
27) Of all things that wisdom provides for living one’s entire life in happiness, the greatest by far is the possession of friendship.
28) The same conviction which inspires confidence that nothing terrible lasts forever, or even for long, also enables us to see that in the midst of life's limited evils, nothing enhances our security so much as friendship.
29) Among desires some are natural and necessary, some natural but not necessary, and others neither natural nor necessary, but due to baseless opinion.
30) Those natural desires which create no pain when unfulfilled, though pursued with an intense effort, are also due to baseless opinion; and if they are not dispelled, it is not because of their own nature, but because of human vanity.
31) Natural justice is the advantage conferred by mutual agreements not to inflict nor allow harm.
32) For all living creatures incapable of making agreements not to harm one another, nothing is ever just or unjust; and so it is likewise for all tribes of men which have been unable or unwilling to make such agreements.
33) Absolute justice does not exist. There are only mutual agreements among men, made at various times and places, not to inflict nor allow harm.
34) Injustice is not an evil in itself, but only in consequence of the accompanying fear of being unable to escape those assigned to punish unjust acts.
35) It is not possible for one who secretly violates the provisos of the agreement not to inflict nor allow harm to be confident that he won’t get caught, even if he has gotten away with it a thousand times before. For up until the time of death, there is no certainty that he will indeed escape detection.
36) Justice is essentially the same for all peoples insofar as it benefits human interaction. But the details of how justice is applied in particular countries or circumstances may vary.
37) Among actions legally recognized as just, that which is confirmed by experience as mutually beneficial has the virtue of justice, whether it is the same for all peoples or not. But if a law is made which results in no such advantage, then it no longer carries the hallmark of justice. And if something that used to be mutually beneficial changes, though for some time it conformed to our concept of justice, it is still true that it really was just during that time – at least for those who do not fret about technicalities and instead prefer to examine and judge each case for themselves.
38) Where, without any change in circumstances, things held to be just by law are revealed to be in conflict with the essence of justice, such laws were never really just. But wherever or whenever laws have ceased to be advantageous because of a change in circumstances, in that case or time the laws were just when they benefited human interaction, and ceased to be just only when they were no longer beneficial.
39) He who desires to live in tranquility with nothing to fear from other men ought to make friends. Those of whom he cannot make friends, he should at least avoid rendering enemies; and if that is not in his power, he should, as much as possible, avoid all dealings with them, and keep them aloof, insofar as it is in his interest to do so.
40) The happiest men are those who enjoy the condition of having nothing to fear from those who surround them. Such men live among one another most agreeably, having the firmest grounds for confidence in one another, enjoying the benefits of friendship in all their fullness, and they do not mourn a friend who dies before they do, as if there was a need for pity.
Translated by Erik Anderson, in his memory