1. Knowledge: Knowledge Is Possible; The Faculties Given Us By Nature (And Not "Reason") Are How We Measure Truth

Before we can productively discuss what exists (Physics) or how to live (Ethics), we must first settle a prior question: how is knowledge itself possible? What are the reliable sources of information about reality, and how do we tell genuine knowledge from mere opinion? This is why Epicurus placed this subject, which the school called the Canon, first and treated it as the foundation on which everything else stands – get this wrong, and anything built on top of it is built on sand.

Therefore Canonics is not a preliminary formality to get through before the "real" subjects begin. It is the foundation that makes the rest of the philosophy possible at all.

Knowledge Of Things That Are Relevant To Us Is Attainable If We Pursue That Knowledge In Ways Consistent With The Nature of The Universe

Therefore we must not pursue knowledge in ways that are contrary to the facts of nature, and of these erroneous methods of pursuing nature there are three primary errors:
It is an error to think that knowledge must be based on "forms" or "models" or "ideals" that are held to exist in another reality. This error leads to the belief that the things we experience around us originate from and are to be understood according to those nonexistent "forms." (Plato and others)
It is an error to think that knowledge must be based on "essences" that are held to exist as a part within the things we experience around us, This error leads to the belief that the things we experience around us originate and are to be understood according to these nonexistent "essences." (Aristotle and others)
It is an error to think that knowledge must be based on divine revelation or by reference to "gods" or "prime movers" who create all things according to their divine will. This error leads to the belief that the things around us originated and are to be understood according to religion. (Judaism, Christianity, and others)
Lucretius states the underlying reason all three errors fail: nothing exists that is not body or the void those bodies move through, so no third category – not a Platonic form, not an Aristotelian essence, not a revealed divine plan – is left over for knowledge to be grounded in.
“For all things that have a name, you will find either properties linked to these two things [body and void] or you will see them to be their accidents. That is a property which in no case can be sundered or separated without the fatal disunion of the thing... And it must be avowed that no man feels time by itself apart from the motion or quiet rest of things.”
Lucretius, De Rerum Natura I.449ff, tr. Bailey
Therefore we must pursue knowledge by studying the facts of nature, which means that we must remember that we ourselves, as well as the subject of our knowledge, and our means of considering that knowledge, derive from (1) the eternal properties of the elementary matter, and (2) the temporary qualities of the bodies that are formed by the combinations of elementary matter and void.
Therefore our pursuit of knowledge must be based only on the faculties of observation that Nature has brought into existence through the movement of the elementary matter and the bodies that they form.

The Faculties of Observation Provided By Nature Are The Tools By Which We Measure Truth. These are collectively referred to as the Epicurean "Canon of Truth."

Diogenes Laertius gives the concise, canonical statement of this whole list:
“Epicurus, in the Canon ('Yardstick'), says that sensations, preconceptions and feelings are the criteria of truth. The Epicureans add the 'focusings of thought into an impression.'”
Diogenes Laertius 10.31, L&S-THP
The Faculty of Seeing is an inborn capacity which allows us "to see," producing something that we call "a sight." An example of the field in which this faculty operates is that of "seeing the sun." The faculty of sight reports to our minds exactly what it perceives without error or added opinion. But when we speak of "the act of seeing" we are generally including also the thing seen, or "a sight." It is important to keep in mind the difference between "the faculty of seeing" and "a sight." What our mind concludes about the significance of what it perceives as seen -- that is, about its total effect on ourselves, about the nature of the object, and about the conditions of observation which exist when we perceive any particular "sight" -- is an evaluative process that is subject to error. The necessity of considering these factors does not negate the value of the faculty of sight, but requires that we work to understand each experience of seeing so as to accurately understand its significance.
This distinction between a true sensation and a mistaken judgment about it has a very direct modern illustration. A convincingly faked video or image can be genuinely seen – the sensation of seeing it is not itself false – but the further judgment about what the image actually depicts or means can still be entirely wrong. The fix, as always, is not to distrust the faculty of sight itself, but to test the judgment against further evidence before accepting it. Lucretius makes exactly this point about sensation and added opinion:
“...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.”
Lucretius 4.462, tr. Bailey
The Faculty of Hearing is an inborn capacity which allows us "to hear," producing something that we call "a sound." An example of the field in which this faculty operates is that of "hearing a song." The faculty of hearing reports to our minds exactly what it perceives without error or added opinion. But when we speak of "the act of hearing" we are generally including also the thing heard, or "a sound." It is important to keep in mind the difference between "the faculty of hearing" and "a sound." What our mind concludes about the significance of what it perceives as heard -- that is, about its total effect on ourselves, about the nature of the object, and about the conditions of observation which exist when we perceive any particular "sound" -- is an evaluative process that is subject to error. The necessity of considering these factors does not negate the value of the faculty of hearing, but requires that we work to understand each experience of hearing so as to accurately understand its significance.
The Faculty of Touching is an inborn capacity which allows us "to touch," producing something that we call "a touch." An example of the field in which this faculty operates is that of "touching a statue." The faculty of touch reports to our minds exactly what it perceives without error or added opinion. But when we speak of "the act of touching" we are generally including also the thing touched, or "a touch." It is important to keep in mind the difference between "the faculty of touching" and "a touch." What our mind concludes about the significance of what it perceives as touched -- that is, about its total effect on ourselves, about the nature of the object, and about the conditions of observation which exist when we perceive any particular "touch" -- is an evaluative process that is subject to error. The necessity of considering these factors does not negate the value of the faculty of touch, but requires that we work to understand each experience of touching so as to accurately understand its significance.
The Faculty of Tasting is an inborn capacity which allows us "to taste," producing something that we call "a taste." An example of the field in which this faculty operates is that of "tasting a fig." The faculty of taste reports to our minds exactly what it perceives without error or added opinion. But when we speak of "the act of tasting" we are generally including also the thing seen, or "a taste." It is important to keep in mind the difference between "the faculty of tasting" and "a taste." What our mind concludes about the significance of what it perceives as tasted -- that is, about its total effect on ourselves, about the nature of the object, and about the conditions of observation which exist when we perceive any particular "taste" -- is an evaluative process that is subject to error. The necessity of considering these factors does not negate the value of the faculty of tasting, but requires that we work to understand each experience of tasting so as to accurately understand its significance.
The Faculty of Smelling is an inborn capacity which allows us "to smell," producing something that we call "an odor" or "a smell." An example of the field in which this faculty operates is that of "smelling a flower." The faculty of smelling reports to our minds exactly what it perceives without error or added opinion. But when we speak of "the act of smelling" we are generally including also the thing smelled, or "a smell." It is important to keep in mind the difference between "the faculty of smelling" and "a smell." What our mind concludes about the significance of what it perceives as smelled -- that is, about its total effect on ourselves, about the nature of the object, and about the conditions of observation which exist when we perceive any particular "smell" -- is an evaluative process that is subject to error. The necessity of considering these factors does not negate the value of the faculty of smell, but requires that we work to understand each experience of smelling so as to accurately understand its significance.
The Faculty of Anticipations is an inborn capacity which allows us "to anticipate" an abstract relationship, producing something that we call "an anticipation." An example of the field in which this faculty operates is that of "classifying justice." The faculty of anticipation reports to our minds exactly what it perceives without error or added opinion. But when we speak of "the act of anticipating" we are generally including also the abstraction perceived, or "an anticipation." It is important to keep in mind the difference between "the faculty of anticipations" and "an anticipation." What our mind concludes about the significance of what it perceives as anticipated -- that is, about its total effect on ourselves, about the nature of the abstract relationship, and about the conditions of observation which exist when we perceive any particular "anticipation" -- is an evaluative process that is subject to error. The necessity of considering these factors does not negate the value of the faculty of anticipation, but requires that we work to understand each experience of anticipating so as to accurately understand its significance.
The Faculty of "Feelings" or "Passions" is a collective name given in Epicurean Philosophy to the Faculty of Pleasure and Pain - the inborn capacity which allows us to experience Pleasure and Pain. This faculty operates in conjunction with each of the other faculties listed above, and concerns not only physical objects but also mental thoughts. This faculty operates in all fields of perception and conception of which men are capable, from the most simple experience of tasting or touching to the most complicated experience of mental consideration of the most complicated or exalted emotions and abstractions. This is the only faculty given to living beings by Nature for the direct experience of anything - from the most simple to the most complex - as desirable or undesirable. It is because the Faculty of Pleasure is our only natural faculty for experiencing that which is desirable in life that Lucretius refers to "Divine Pleasure, Guide of Life." But the great mistake of many philosophers is to condemn pleasure and extol pain. This mistake occurs even though no one rejects, dislikes or avoids pleasure in itself, simply because it is pleasure. This mistake occurs because those who do not pursue pleasure rationally encounter consequences that are extremely painful. Another reason for this mistake is that people fail to understand that mental pleasure and pain is often much more important in life and intense than physical pleasure and pain. It is therefore very important to keep in mind that the Faculty of Pleasure and Pain must be considered and employed wisely, using using the same framework as the other faculties:
The "Faculty of Pleasure" allows us "to experience as pleasing," producing something that we call "a pleasure." The faculty of pleasure reports to our minds exactly what it perceives without error or added opinion. But when we speak of "the act of experiencing pleasure" we are generally including also the thing perceived as pleasing, or "a pleasure." It is important to keep in mind the difference between "the faculty of pleasure" and "a pleasure." What our mind concludes about the significance of what it perceives as pleasing -- that is, about its total effect on ourselves, about the nature of whatever we find to be pleasing, and about the conditions of observation which exist when we perceive any particular "pleasure" -- is an evaluative process that is subject to error. The necessity of considering these factors does not negate the value of the faculty of Pleasure, but requires that we work to understand each experience of pleasure so as to accurately understand its significance.
The "Faculty of Pain" allows us "to experience as painful," producing something that we call "a pain." The faculty of pain reports to our minds exactly what it perceives without error or added opinion. But when we speak of "the act of experiencing pain" we are generally including also the thing perceived as painful, or "a pain." It is important to keep in mind the difference between "the faculty of experiencing pain" and "a pain." What our mind concludes about the significance of what it perceives as painful -- that is, about its total effect on ourselves, about the nature of whatever we find to be painful, and about the conditions of observation which exist when we perceive any particular "pain" -- is an evaluative process that is subject to error. The necessity of considering these factors does not negate the value of the faculty of Pain, but requires that we work to understand each experience of pain so as to accurately understand its significance.

The Epicurean Canon Compared To Its Ancient Rivals

The Stoics also claimed that certain knowledge was possible, but they built a very different, and far less usable, apparatus to get there. Where the Epicurean Canon rests on faculties present in every creature from birth, the Stoics required a special kind of impression – one supposedly so clear and forceful that it certified its own accuracy – and held that only a fully rational, perfected mind could reliably tell such an impression apart from a false one that felt exactly the same. Chrysippus, one of the school's own leaders, is reported to have said that such a perfected mind, a Stoic sage, was harder to find than a phoenix.
Cicero preserves the Stoic founder Zeno's own illustration of this system: an open hand for a mere impression, fingers curling for assent, a closed fist for the "grasp" that supposedly amounts to certainty – and then Zeno would clasp that fist with his other hand and squeeze, saying that this, and only this, was genuine knowledge, "which none but a wise person possessed."
Therefore the Stoic standard, however impressive as a piece of theater, effectively locks certainty behind an ideal of perfected reason that almost no living person actually reaches, leaving ordinary people permanently short of real knowledge. The Epicurean standard requires no such achievement: the senses, the anticipations built through ordinary experience, and the feelings of pleasure and pain are available immediately, to every person, by nature, without any special training at all.

We Must Apply The Proper Standard of "Certainty" In Applying the Canon of Truth.

Therefore we do not seek to understand everything equally well, nor do we expect to understand that which is not possible for us to understand. We seek only to have firm convictions about those things which are necessary for our peace of mind.
Letter to Pythocles: In the first place, remember that, like everything else, knowledge of celestial phenomena, whether taken along with other things or in isolation, has no other end in view than peace of mind and firm convictions. We do not seek to wrest by force what is impossible, nor to understand all matters equally well, nor make our treatment always as clear as when we discuss human life or explain the principles of physics in general—for instance, that the whole of being consists of bodies and intangible nature, or that the ultimate elements of things are indivisible, or any other proposition which admits only one explanation of the phenomena to be possible.

Those Who Deny That Any Knowledge Is Possible Refute Themselves By The Very Act of Arguing

A different challenge comes not from a rival standard of certainty, but from the claim that no standard is possible at all. The philosophers of the Academy who followed this path (Academic Skepticism) held that since certainty can never be guaranteed, judgment about anything should simply be suspended. A more extreme group (Pyrrhonism), tracing its approach back to a philosopher who had traveled with ascetic teachers as far as India, went further still and counseled suspending judgment even about the value of suspending judgment itself – and both groups must be kept distinct, since they are not the same position and do not fail for exactly the same reason.
Both groups, however, share the same fatal defect: the claim that nothing can be known is itself offered as something known, and refutes itself the moment it is stated. And, strikingly, both groups end up claiming that the goal of their own approach is a state of tranquility, of freedom from disturbance – even though, on their own premises, they cannot know with any confidence that this tranquility is actually good for them, or achievable, or different from mere resignation. A position that cannot certify its own goal is not a serious rival account of how to live; it is the abandonment of the entire project, dressed up as its completion.
Lucretius states this self-refutation directly:
“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… You will find that the preconception of true has its origin in the senses, and that the senses cannot be refuted… 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… Hence you will find that the entire battalion of words which has been marshaled and armed against the senses is futile.”
Lucretius, De Rerum Natura 4.469–521
Therefore we must reject any philosophy, ancient or modern, that treats the impossibility of knowledge as a sophisticated discovery rather than what it actually is: a refusal to use the very faculties that make it possible to live, to judge, and to choose at all.

"Reason" and "logic" are not faculties of observation. Properly understood, "reason" and "logic" are only the names we give to the important process of accurately comparing, contrasting, analogizing, and evaluating the data obtained by our faculties of observation.

Therefore there is an important but subordinate role for "reason" and "logic" in classifying conclusions about what is "true" and "false."
PD 16. In but few things chance hinders a wise man, but the greatest and most important matters reason has ordained and throughout the whole period of life does and will ordain.
Diogenes Laertius - Biography of Epicurus: Every sensation, he says, is devoid of reason and incapable of memory; for neither is it self-caused nor, regarded as having an external cause, can it add anything thereto or take anything therefrom. Nor is there anything which can refute sensations or convict them of error: one sensation cannot convict another and kindred sensation, for they are equally valid; nor can one sensation refute another which is not kindred but heterogeneous, for the objects which the two senses judge are not the same; nor again can reason refute them, for reason is wholly dependent on sensation; nor can one sense refute another, since we pay equal heed to all. And the reality of separate perceptions guarantees the truth of our senses. But seeing and hearing are just as real as feeling pain. Hence it is from plain facts that we must start when we draw inferences about the unknown. For all our notions are derived from perceptions, either by actual contact or by analogy, or resemblance, or composition, with some slight aid from reasoning.
CIcero's On Ends: Logic, on which your [Stoic} school lays such stress, he [Epicurus] held to be of no effect either as a guide to conduct or as an aid to thought. Natural Philosophy he deemed all-important. This science explains to us the meaning of terms, the nature of predication, and the law of consistency and contradiction; secondly, a thorough knowledge of the facts of nature relieves us of the burden of superstition, frees us from fear of death, and shields us against the disturbing effects of ignorance, which is often in itself a cause of terrifying apprehensions; lastly, to learn what nature's real requirements are improves the moral character also. Besides, it is only by firmly grasping a well-established scientific system, observing the Rule or Canon that has fallen as it were from heaven so that all men may know it—only by making that Canon the test of all our judgments, that we can hope always to stand fast in our belief, unshaken by the eloquence of any man. On the other hand, without a full understanding of the world of nature it is impossible to maintain the truth of our sense-perceptions. Further, every mental presentation has its origin in sensation: so that no certain knowledge will be possible, unless all sensations are true, as the theory of Epicurus teaches that they are. Those who deny the validity of sensation and say that nothing can be perceived, having excluded the evidence of the senses, are unable even to expound their own argument. Besides, by abolishing knowledge and science they abolish all possibility of rational life and action. Thus Natural Philosophy supplies courage to face the fear of death; resolution to resist the terrors of religion; peace of mind, for it removes all ignorance of the mysteries of nature; self-control, for it explains the nature of the desires and distinguishes their different kinds; and, as I showed just now, the Canon or Criterion of Knowledge, which Epicurus also established, gives a method of discerning truth from falsehood.
Therefore the assertions of reason must constantly be tested against the data obtained from our faculties of observations:
Assertions of "reason" and "logic" should be concluded to be "true" when clear available evidence supports the conclusion, and no clear evidence contradicts the conclusion.
Assertions of "reason" and "logic" should be concluded to be "false" when clear available evidence contradicts the conclusion, and no clear evidence supports the conclusion.
Assertions of "reason" and "logic" which are not based on data from our faculties of observation, or on which the data from our faculties of observations is unclear, must not be considered "true," but considered "speculative" at best, and we must "wait" for additional evidence before concluding that the assertion is either true or false.