035

Letter to Herodotus

A Summary of Physical Nature

Reasons for the Letter

Epicurus to Herodotus,

Greetings,

For those, Herodotus, who can neither master all my physical doctrines nor digest my lengthier books On Nature, I have written a summary of the whole subject in enough detail to enable them to easily remember the most basic points, and thereby grasp these important and irrefutable principles entirely on their own, to whatever degree they take up the study of physics.  Even those who have thoroughly learned the entire system must be able to summarize it, for an overall understanding is more often needed than a specific knowledge of details.
 

036

We must therefore continually refresh our memory with these principles, in order to retain the general outline.  Moreover, once the basic points have been mastered, specific knowledge of details can be learned more easily.  But the most important benefit of specific knowledge, even for the fully-initiated, is that it reinforces a general understanding of the fundamental principles.  Indeed, it is impossible to reap the rewards of further studying the universe, unless one can comprehend in simple terms all that could be expressed in great detail.
 

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Since this pattern of study is useful to everyone concerned, I, who devote myself continuously to the subject and who am most at peace by living this sort of life, have prepared for you a summary and outline of my entire teachings.

Rules of Procedure

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First, Herodotus, we must use clearly defined terms, so that when we refer to them, we can make judgments upon particular inquiries, problems, or opinions, rather than to remain undecided after endless arguments devoid of meaning.

Thus, we must accept, without further proof, the first mental image each word conjures up, if we are to have any standard to refer a particular inquiry, problem, or opinion.  Next, we must conduct all our investigations based on the testimony of our senses, feelings, and all other valid criteria.  In this way, we shall have some sign by which to make inferences about things awaiting confirmation <by the testimony of our senses> and also about things <that will always remain> hidden from our senses.

Basic Aspects of Existence

039

Having made this distinction, let us now consider what is not directly evident to our senses:

Nothing comes into existence from non-existence.   For if that were possible, anything could be created out of anything, without requiring seeds.  And if things which disappear became non-existent, everything in the universe would have surely vanished by now.  But the universe has always been as it is now, and always will be, since there is nothing it can change into.  Nor is there anything outside the universe which could infiltrate it and produce change.
 

040

The universe is made up of bodies and void.  That bodies exist is obvious to anyone’s senses.  We may also make inferences about things hidden from our senses, as I have noted above, only from signs that our senses can detect, and this is how we infer the void.  For if the void, which we also call place, room, and intangible substance, did not exist, bodies would have no place to be or anywhere to move through – but they are clearly seen to be moving.  Beyond these constituents [body and void] nothing else is conceivable by any means.  Both are regarded as whole substances – not attributes of them.
 

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041

Compounds are collections of many elements; the primary bodies are the elements themselves.  The latter must be uncuttable {atomic}, and permanent – otherwise all things would crumble into non-existence.   Some elements must be strong enough to survive the dissolution of compounds; these are fully solid by nature, incapable of dissolution to any degree.  So these primary bodies must be uncuttable bodies.
 

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042

The universe is infinite.  For that which is finite has an outmost edge, and an outmost edge can only be found in comparison to something beyond it <but the universe cannot be so compared>, hence, since it has no outmost edge, it has no limit; and since it has no limit, it must be unlimited and infinite.  Indeed, the universe is infinite in two aspects: by the number of bodies it contains and by the extent of the void.  For if the void were infinite but the bodies finite, the bodies would go careening through the infinite void and never stay put, owing to the lack of other bodies to hinder and coral them by colliding with them.  And if the void were finite, there would be no room for infinite bodies.
 

043

The atoms have a unimaginable variety of shapes.  Since all compounds are formed by (and dissolve into) solid atomic bodies, the many varieties of compounds that exist can only arise from an unimaginable number of atomic shapes.  But while the number of atoms of each shape is utterly infinite, the number of shapes is not utterly infinite, just unimaginably many, <otherwise atoms would have an infinite range of sizes, which would defy observation>.
 

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044

The atoms are in constant motion throughout eternity.   {some text missing}  Some get separated by great distances from each other.  Others oscillate in one place whenever they happen to get entangled into a compound, or surrounded by a compound.  It is the nature of both bodies and void which allows this oscillatory motion.  For the bodies, being solid, rebound on collision to whatever distance their entanglement allows them, while the void offers no resistance in the intervening space.  This may continue until at last the repeated shocks bring on the dissolution of the compound.  There is no beginning to all these motions; the atoms and void are eternal.
 

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045a

These above points, if remembered, should suffice as an outline for developing an understanding of the basic aspects of existence.
 

{Misplaced text 45b moved to 73 below}

The Mechanics of Sight

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047b

Images abound which resemble the outlines of shapes.  These husk-like emanations [of atoms] are the thinnest things perceivable.  Their existence is possible because in the space around us, appropriate conditions exist to accommodate their hollowness and thinness, thus preserving the same orientation and shape as the surfaces they are thrown off from.  These outlines we call ‘images.’ {Misplaced text 46b & 47a moved to 61 below}  That the images are exquisitely thin is uncontested by anything evident, for they move with insuperable speed and arrive at our eyes together.  We thus know that their passage is met with little or no resistance, whereas many, indeed all, compounds suffer immediate collisions.
 

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The creation of images happens as fast as thought.  They are cast off continuously, in outline form, from the surfaces of compounds – obviously without wasting away, thanks to reciprocal replenishment.  The orientation and arrangement of atoms in an image usually correspond to the emitting surface without distortion.  But sometimes images get combined in midair.  This fusion happens quickly since no interactions are required within the volume of space contained by their outlines.  There are also other ways in which images are produced.  None of these facts are contested by our sensations, when we consider how sensation brings us coherent visions of objects around us.
 

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Sight and perception are due to images received directly from the surfaces of objects.  We would not perceive their shape or color very effectively if their emanations were actively mediated by the intervening air {as Democritus believed}, or by means of light-rays or some sort of flowing current directed to us by them.  Rather, we are directly impacted by husks from the objects, which share the color and shape of their source, but are thin enough to penetrate our senses.  Since these are cast off in rapid succession (in sympathy with the vibrations of atoms in the depths of the object), they present an uninterrupted image and preserve their relationship to the source.

Visual Impressions versus Opinions

051

The mental picture formed by intense visual scrutiny or concentrated thought, is true.  Because this sort of picture is created by the continuous impact of  imagery, or by the actual residue it leaves behind, the shapes or properties of an external object are thereby correctly revealed.

Falsehood and error reside in opinion.  When <an image awaiting> confirmation (or at least non-contradiction) from further sensory evidence fails to be so confirmed <or is contradicted>, this is due to the embellishment of the image by opinion.  {text uncertain here, perhaps also some missing} ...

For the mental pictures that come to us either in sleep, or by concentration, or by the other instruments of judgment, would not have such similarity to those things we deem to truly exist if there were not some kind of flow of material actually coming to us from the objects.  And error would not exist if we did not also permit within ourselves some other activity similar <to the purposeful apprehension of mental images>, yet different.  It is through this other activity {i.e., forming opinions} that, when unattested or contested, produces falsehood, and if attested or uncontested, truth. 
 

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This principle too, then, is important to maintain.  Otherwise, the criteria based on self-evident impressions would be destroyed and falsehood would be taken as well-established as truth, throwing everything into confusion.

Hearing

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Hearing results from a sort of current.   It may come from a person who speaks, or an object that rings, bangs, or produces any sort of auditory sensation.  This current is dispersed into particles, all alike, preserving their common relationship with a well-defined continuity extending all the way back to their origin.  When hearing occurs, the source is usually recognized; failing that, it at least reveals that something is out there.  Without this common relationship stemming from the source, there would not be such awareness.  We should not believe that the air itself is shaped by the spoken word or sound, for the nature of air is hardly shapeable.  Rather, the effort of speaking squeezes out certain particles in a breath-like stream that produces auditory sensations in the person we are speaking to.

Smell

The sense of smell, like hearing, also depends on a current.  Here again, certain particles flow away from an object that are suitably-sized to pass into this senses.  Some kinds of smells are disharmonious and unwelcome, others harmonious and welcome.

Atomic Properties

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Atoms only have shape, weight, size, and attributes of shape {e.g., smoothness or roughness}.  While qualities of compounds change, the atoms do not change at all, since something solid and indestructible must persist in order to make change possible. Change results from rearrangements of certain particles – or from their addition and removal (but never to or from non-existence). Hence these particles are interchangeable, but unchangeable – their own particular weights and shapes persist.
 

055

Even when objects we ordinarily see are chipped away at, they still retain shape, weight, {and size} – while other qualities do not remain but vanish entirely.  The properties which endure suffice for the variety of compounds in nature; it is necessary that at least these properties remain and not be annihilated.
 

056

Atoms differ in size, but are not of every size.  We must not think otherwise, lest the visible world prove us wrong.  The evidence of our differing feelings and sensations is best explained if atoms differ in size, but they need not be of every size in order to account for qualitative differences that we perceive.  If atoms were of every size, some would have to be large enough to see.  Clearly this isn’t so, and it’s impossible to suppose how an atom might become visible.

Atomic Parts

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No finite body has infinite parts, and all parts must have a lower limit to size.  We must reject the idea that something finite can be cut smaller and smaller forever – for then all physical objects would be brittle and we could, in concept, exhaust any compound {by infinite cuts} and thereby completely annihilate it.   But we also cannot suppose there are infinite parts in a finite thing, for this raises an immediate problem: how can something containing infinite parts itself be finite?  Each part must extend to some size, and however small they may be, an infinity of them would have to extend to infinite size.  But a finite body has a visible extremity – even if we can't isolate it.  We may suppose the outermost part has a similarly-sized neighboring part, and likewise in sequence, but not without end.
 

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Consider the smallest width we can possibly see: it’s both like and unlike a span.  While it seems to have properties in common with an extended object, it has no distinguishable parts.  If we attempt to distinguish parts – one on this side, the other on that – neither of them can be visibly smaller than the whole minimum.  All we can do is inspect the minima in sequence and we neither find them all in the same place, nor can we find the places where they touch each other.  Yet, in their own peculiar way, they build size – the larger the size, the more minima there are; the smaller the size, the fewer.
 

059

The same description applies to the smallest atomic size.  Obviously, the smallest part of an atom is much smaller than the smallest width we can see.  But here again we can follow the same analogy as we did with our claim that the atom has size: we draw the analogy from the scale of seeable things.  Hence, the atomic minima must be regarded as fixed units, which may serve, at least in our imagination, as a means of reckoning atomic size.  But this is as far as the analogy can go.  We should not take it so far as to think that atoms can be constructed or changed by arranging or rearranging atomic minima, because it is impossible for atomic minima to be moved individually.

Atomic Motion

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There is no top and bottom in infinite space, but up and down are still meaningful.  For wherever we stand it is possible to project a line above our heads, or below our feet, stretching to infinity.  It is possible to do both without confusing each direction with the other.  Therefore it is also possible to regard one type of motion as upward to infinity, and another type as downwards to infinity.  Even if that which moves from where we are to the places above our heads arrives countless times at the feet of those above, or in the other case, at the heads of those below, the two motions are still opposite.
 

061

Atoms move equally fast through the void when nothing collides with them.  Large and heavy ones move no faster than small and light ones, nor vice a versa, as long as nothing obstructs them.  Their movements are neither made quicker when deflected upwards or sideways, nor when they fall downwards due to their respective weights.  The atom will traverse any kind of trajectory with the speed of thought as long as the motion caused in either of these ways maintains itself – that is, until the atom is either re-deflected by another collision, or its own weight counteracts the force of a previous collision.
 

046b

Motion through the void may traverse any ordinary distance in an extraordinarily short time, because the lack of obstruction from colliding bodies.  Only through collision and non-collision can atomic motion resemble “slow” and “fast.”
  

047a

On the other hand, a moving body cannot arrive at several places at once in the shortest conceivable period of time. That is unthinkable.  But when in a perceivable period of time a body arrives along with others from some point or other in the infinite, the distance covered will be extraordinary.  If it were otherwise, collisions would have been involved – though we still allow some limit to speed of motion as a result of non-collision. This too is a useful principle to grasp.
 

062

Atoms also move equally fast  in compounds, despite anything said to the contrary.  Compounds, and the atoms within them, do move in a single direction in the shortest perceivable period of time.  But in the shortest conceivable period of time, the atoms are going every direction, owing to their frequent collisions.  Only the continuity of their collective motion is slow enough to be seen.

The opinion (added to what the senses cannot perceive) that “there will be continuity of motion in conceivable periods of time” is not true in the case of atoms.  Only what is grasped by the careful use of the senses or by the mental apprehension of concepts is wholly true.

The Soul 

063

The soul is a fine-structured material distributed throughout the body.  Our sensations and feelings provide the strongest confirmation for this.  It resembles a wind in some respects and heat in others.  But  its fine structure makes it greatly different from both – and this is what unites its feelings with the entire body.  All this is demonstrated by the soul’s powers: its feelings, its rapid action, its thought processes, and all of its faculties which we are deprived of upon death.
 

064

The soul is primarily responsible for sensation.  Yet, it would not have acquired sensation if it were not contained in some way by the rest of the body.  The rest of the body, having furnished the proper setting for experiencing sensation, is also given some capacity for sensation from the soul – though not all the capacity of the soul.  That is why the rest of the body does not have sensation when the soul has been separated from it.  For the body never had such capacity in and by itself; it made sensation possible for something else [the soul], which came into existence along with it.  The soul, thanks to the mechanisms of the body, at once produces its own power to experience sensation while returning a share of this power to the body, as I have said, because of their close contact and united feelings.
 

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Sensation is never lost when the soul remains, even if other parts of the body are lost.  Indeed, even if part of the soul is lost along with the part of the body  that enclosed it, then as long as part of the soul remains, it will still experience sensation.

But sensation is lost when the body remains and the soul has been lost – no matter how small the atoms comprising the soul may be.

When the whole body is destroyed, it no longer possesses sensation, because the soul is dissolved and no longer has the same powers and motions.  For whenever the body holding the soul is no longer able to confine and contain it, we cannot think of the soul as still experiencing sensation, since it would no longer have the use of the appropriate mechanisms.
 

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067

Those who say that the soul is incorporeal are talking nonsense.  The usage of the word ‘incorporeal’ can only be applied to what is incorporeal in essence: the void.  But the void can neither act nor be acted upon; it merely allows bodies to move through itself.  For if that were so, it would be unable to act or be acted upon in any way – yet, we clearly see the soul is capable of both.
 

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If, as was said at the beginning, you explore your feelings and sensations while considering these points about the soul, you will find enough of a basis in this outline to enable you to discover the details with certainty.

Properties and Accidents

069

Shapes, colors, sizes, weights, etc., are properties pertaining to bodies.  This is true for bodies in general as well for perceivable ones, where properties are recognized by direct sensation.  Properties are not substances themselves – it is inconceivable to think of them separate from the things they are properties of.  Nor are they non-existent, nor are they incorporeal things attached to the body, nor are they parts of a body.
 

A body as a whole gets its enduring characteristics from the combination of all its properties.  This does not mean that these properties come together and form the body in the way that a larger body is formed by smaller parts (e.g., by primary bodies or compounds smaller than the whole).  We merely mean, as I have said, that the whole body gets its enduring characteristics from the presence of the properties within.  These properties have their own way of being perceived and distinguished (together with the body – never separate from it); it is because of this all-inclusive notion of the body as a whole that it is so recognized.
 

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Now it often also happens that temporary qualities accompany body – accidents.  They too do not exist by themselves nor are they incorporeal.  Accidents are neither like the whole, which we grasp collectively as a body, nor are they like the enduring characteristics essential to a body.  Any accident can be recognized by the appropriate senses, along with the compound to which it belongs; but we see a particular accident only when it is present with the body, since accidents are temporary.
 

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We must not deny the self-evident reality of accidents, just because they do not have the nature of the whole (i.e., the body, of which it becomes an attribute) nor the nature of permanent attributes.  Nor should we think of them as entities having independent existence.  We should think of  accidents of bodies as just what they seem to be and not as permanent attributes nor as existing independently.  They are seen in just the way that our senses discern them.
 

072

Time is something else that must also be carefully considered.  We cannot investigate time in the same way we can for things that can be seen in objects and visually apprehended by the mind.  Instead, we must reason by analogy from the experience of what we call “a long time” versus “a short time.”
 

073a

We do not need better descriptions of time; we may use those already at hand.  Nor do we need to assert that this unique entity is based on something else of the same nature, as some indeed do.  It is only important to consider the things we associate with time and the ways in which we measure it.  This requires no elaborate demonstration – only a review of the facts.  We associate time with days and nights (and fractions thereof), and likewise with the presence and absence of feelings, and with motions and rests.  Thus we recognize that the very thing we call time is, in a special sense, an accident of accidents.

World-Systems

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045b

The number of world-systems is infinite.   These include worlds similar to our own {which means the Earth plus the sky and all its celestial bodies} and dissimilar ones.  For the atoms, being infinitely many, as already proved, travel any distance, and those which are able to form a world are not exhausted by the formation of one world or by any finite number of them – both ones like ours or other kinds.  So nothing prevents there being an infinite number of worlds.
 

073b

{World-systems, like all compounds, are perpetually created and destroyed}.  The world-systems, and every observable compound, come into being from the infinite.  All such things, large or small, have been separated off from it as a result of individual entanglements.  And all will disintegrate back into it – some faster, some slower, and by differing causes.
 

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{text missing} Though the creation of worlds is inevitable, we must not suppose that each necessarily has a single shape <or every possible shape...>
 

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{text missing} <... Moreover, with regard to living things,> it cannot be proven that the seeds from which animals, plants, and other things originate are not possible on any particular world-system.

Natural History

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075

In their environment, primitive men were taught or inspired by instinct to do many kinds of things, but reason later built upon what had been begun by instinct.  New discoveries were made – faster among some people, slower among others.  In some ages and eras <progress occurred by great leaps>, in others by small steps.
 

076

Words, for instance, were not initially coined by design.  Men naturally experienced feelings and impressions which varied in the particulars from tribe to tribe, so that each of the individual feelings and impressions caused them to vocalize something in a particular way, in accordance also with differing racial and environmental factors.  Later, particular coinages were made by consensus within the individual races, so as to make the distinctions less ambiguous and more concise.  Men who shared knowledge also introduced certain abstractions, and brought words for them into usage – sometimes making utterances spontaneously, and other times choosing words rationally.  This is mainly how they achieved self-expression.

Celestial Phenomena

077

Celestial phenomena do not occur because there is some divinity in charge of them.  No deity could arrange and maintain motions, periods, eclipses, risings, settings, and the like, while at the same time enjoy perfect happiness and immortality.  For trouble, anxiety, anger, and obligation are not associated with blessedness, but rather with weakness, fear, and dependence on others.  Masses of  fire [are not gods];  they have not acquired a state of divine blessedness nor have they undertaken these motions of their own free will.  Whenever we speak of blessedness, we must respect [the true meaning of] its majesty, or else we shall create great turmoil in our souls.  So it stands to reason that when celestial bodies were formed (at the time our world-system was created) the regularity of their motions was fixed.
 

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We must accept the following beliefs:

• The purpose of physics is to correctly identify the causes of phenomena that concern us.

• Our happiness depends on this, and on knowing what celestial bodes really are, and on related facts.

• Anything that suggests conflict or disturbance in divine nature simply cannot occur – there are no two ways about it.

• It is possible for the human mind to reason out that all this is true without qualification.

079

Detailed celestial data do not contribute to the happiness which comes with general knowledge.  Those who have studied settings and risings, periods and eclipses, and the like, but are ignorant of their underlying nature and their causes are subject to the same fears despite what they know – or perhaps even greater fears, because the amazement that follows from studying these phenomena does not reveal their fundamental causes.
 

080

If we find several possible causes for some celestial phenomena, we have not failed to learn enough to secure peace of mind and happiness.  In order to investigate the causes of celestial phenomena (or anything else which cannot be scrutinized up-close) we begin by finding how many ways similar phenomena are produced within the range of our senses.  We must pay no attention to those who fail to recognize any difference between what results from a single cause or from several causes; they forget that these phenomena are only perceived at a distance, and they do not know what circumstances make it possible or impossible to achieve peace of mind.  If we recognize that phenomena may occur in several ways, we shall be no less disturbed than if we knew for sure that a particular phenomenon happens in some particular way.
 

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• They think that celestial bodies are blessed and immortal [i.e., godlike] yet desire, scheme, and act in ways that are incompatible with divine nature.

• They either foresee their deaths as eternal suffering, as depicted in myths, or they fear the very lack of consciousness that accompanies death as if it could be of concern to them.

• They suffer all this not because there is a reasonable basis, but because of their wild imagination; and by not setting a limit to suffering, their level of turmoil matches or exceeds what they would suffer even if there was a reasonable basis.

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Peace of mind comes from having been freed from all this, and by always remembering the essential principles of our whole system of belief.  Thus, we should pay attention to those feelings and sensations which are present within us (both those we have in common with humankind at large, and the particular ones we have in each of ourselves) according to each of the criteria of truth.  Only then shall we pin down the sources of disturbance and fear.  And when we have learned the causes of celestial phenomena and related events, we shall be free from whatever is terrifying to the rest of humankind.

Conclusion

083

Here then, Herodotus, you have the most important points of physics set down in an outline form that is suitable for memorization.  I believe that anyone who masters this much will be made stronger than his fellow men, even without going into a more detailed study.  And one will also be better disposed to understanding many detailed points of our system as a whole [should he elect to do so]; these general principles will be of constant help if he keeps them in mind.  For no matter how far along one is in mastering the details, those who solve their solve their problems with reference to this outline will make the greatest advances in the knowledge of the whole.  And even those who have made less progress can, without oral instruction, quickly review the matters of most importance for peace of mind.