Epicurus to Herodotus, greetings:
For the sake of those, Herodotus, who are unable to work out with precision each and every detail of what we have written on nature and who lack the ability to work through the longer books I have composed, I have myself prepared an adequate summary of the entire system, to facilitate the firm memorization of the most general doctrines, in order that at each and every opportunity they may be able to help themselves in the most important issues, to the degree that they retain their grasp on the study of nature. Even those well advanced in the examination of the universe must recall the outline of the entire system; and this outline is structured according to basic principles. For we frequently need the overall application [of the intellect], but not so often the detailed application.
We must, then, approach those [general points] continually, and get into our memory an amount [of doctrine] sufficient to permit the most vital application [of the intellect] to the facts; moreover, complete precision on detailed points will be discovered if the general outlines are comprehensively grasped and remembered. For even the fully expert [student of physics] gets as the most vital benefit of complete precision the ability to make nimble use of his applications, and this would happen if every point were united in [a set of] simple principles and maxims. For it is not possible to know the concentrated result of our continuous overview of the universe unless one can have in oneself a comprehensive grasp by means of brief maxims of all that might also be worked out in detail with precision.
Since this kind of method is useful to all those who are concerned with the study of nature, I recommend constant activity in the study of nature; and with this sort of activity more than any other I bring calm to my life. That is why I have composed for you this type of summary statement of the basic principles of the entire set of doctrines.
First, Herodotus, we need to have grasped what is denoted by our words,  so that by referring to what they denote we can make decisions about the objects of opinion, investigation, or puzzlement and  so that all of these things will not remain undecided, [as they would] if we tried to give an infinitely long demonstration, and  so that our words will not be empty. For it is necessary that we look to the primary conception corresponding to each word and that it stand in no need of demonstration, if, that is, we are going to have something to which we can refer the object of search or puzzlement and opinion. Again, it is also necessary to observe all things in accordance with one’s sense-perceptions, i.e., simply according to the present applications, whether of the intellect or of any other of the criteria, and similarly [to observe everything] in accordance with our actual feelings, so that we can have some sign by which we may make inferences both about what awaits confirmation and about the non-evident.
After distinguishing these points we must next arrive at a general view about the things which are non-evident. The first point is that nothing comes into being from what is not; for [in that case] everything would be coming into being from everything, with no need of seeds. And if that which disappears were destroyed into what is not, all things would have been destroyed, since that into which they were dissolved does not exist. Further, the totality [of things] has always been just like it is now and always will be. For there is nothing for it to change into. For there exists nothing in addition to the totality, which could enter into it and produce the change.
Moreover,1 the totality is [made up of] bodies and void; for in all cases sense-perception itself testifies that bodies exist, and it is by sense-perception that we must infer by reasoning what is non-evident, as I already said. And if there did not exist that which we call void and space and intangible nature, bodies would not have any place to be in or move through, as they obviously do move. Beyond these two things [viz. bodies and void] nothing can be conceived, either by a comprehensive grasp or analogously to things so grasped, [at least not if we mean] grasped as complete natures rather than as what are termed properties or accidents of these [two] things.
Further, among2 bodies, some are compounds, and some are those things from which compounds have been made. And these are atomic and unchangeable, if indeed they are not all going to be destroyed into not being but will remain firmly during the dissolutions of compounds, being full by nature and not being subject to dissolution in any way or fashion. Consequently the principles of bodies must be atomic natures.
Moreover, the totality is unlimited. For what is limited has an extreme; but an extreme is seen in contrast to something else, so that since it has no extreme it has no limit. But since it has no limit it would be unlimited and not limited.
Further, the totality is unlimited in respect of the number of bodies and the magnitude of the void. For if the void were unlimited and bodies limited, bodies would not come to a standstill anywhere but would move in scattered fashion throughout the unlimited void, since they would lack anything to support them or check them by collision. But if the void were limited, the unlimited bodies would not have a place to be in.
In addition, the bodies which are atomic and full, from which compounds both come to be and into which they are dissolved, are ungraspable when it comes to the differences among their shapes. For it is not possible that so many differences [in things] should come to be from the same shapes having been comprehensively grasped. And for each type of shape there is, quite simply, an unlimited number of similar [atoms], but with respect to the differences they are not quite simply unlimited but only ungraspable.
And3 the atoms move continuously4 for all time, some recoiling far apart from one another [upon collision], and others, by contrast, maintaining a [constant] vibration when they are locked into a compound or enclosed by the surrounding [atoms of a compound]. This is the result of the nature of the void which separates each of them and is not able to provide any resistance; and their actual solidity causes their rebound vibration to extend, during the collision, as far as the distance which the entanglement [of the compound] permits after the collision.
There is no principle for these [entities], since the atoms and the void are eternal.5 If all these points are remembered, a maxim as brief as this will provide an adequate outline for [developing] our conceptions about the nature of what exists.
Moreover, there is an unlimited number of cosmoi, and some are similar to this one and some are dissimilar. For the atoms, which are unlimited (as was shown just now), are also carried away to very remote distances. For atoms of the sort from which a world might come to be or by which it might be made are not exhausted [in the production] of one world or any finite number of them, neither worlds like this one nor worlds unlike them. Consequently, there is no obstacle to the unlimitedness of worlds.
Further, there exist outlines [i.e., images, eidola] which are similar in shape to solids, only much finer than observed objects. For it is not impossible for such compounds to come into being in the surrounding environment, nor that there should be favourable opportunities for the production of hollow and thin [films], nor that effluences should retain the relative position and standing [i.e., order] that they had in the solid objects. These outlines we call ‘images’. Further, since their movement through the void occurs with no conflict from [atoms which] could resist them, it can cover any comprehensively graspable distance in an inconceivably [short] time. For the presence and absence of resistance takes on a similarity to slowness and speed.
The moving body itself, however, cannot reach several places at the same time, speaking in terms of time contemplated by reason; for that is unthinkable. Yet when considered as arriving in perceptible time from any point at all in the unlimited, it will not be departing from the place from which we comprehensively grasp its motion as having come from. For it will be like resistance even if to this point we leave the speed of the movement free from resistance. The retention of this basic principle too is useful.
Next, none of the appearances testifies against [the theory] that the inages have an unsurpassed fineness; and that is why they have unsurpassed sped too, since they find every passage suitably sized for there being so or few [bodies] to resist their flow, whereas there is some [body to resist a large or infinite number of atoms.
In addition, [none of the facts testifies against the claim] that the production of images occurs as fast as thought. For there is a continuous how from the surface of bodies, though it is not obvious from any reduction in bulk because the objects are] refilled [by other atoms]; [and this flow] preserves for quite some time the position and order of the atoms which it had in the solid, even if it is sometimes disrupted; and [two-dimensional] compounds are quickly produced in the surrounding environment, since they do not need to be filled out with depth—and there are certain other ways in which such natures [i.e., compound images] can be produced. None of these [claims] is testified against by the senses, providing one considers the clear facts in a certain way; one will also refer to [the senses] the [fact that] harmonious sets [of qualities] come to us from external objects.
One must also believe that it is when something from the external obiects enters into us that we see and think about their shapes. For external objects would not stamp into us the nature of their own colour and shape via the air which is between us and them, nor via the rays or any kind of flows which move from us to them, as well as [they would] by means of certain outlines which share the colour and shape of the objects and enter into us from them, entering the vision or the intellect according to the size and fit [of the effluences] and moving very quickly; then, for this reason, they give the presentation of a single, continuous thing, and preserve the harmonious set [of qualities] generated by the external object, as a result of the coordinate impact from that object [on us], which [in turn] originates in the vibration of the atoms deep inside the solid object. And whatever presentation we receive by a form of application, whether by the intellect or by the sense organs, and whether of a shape or of accidents, this is the shape of the solid object, produced by the continuous compacting or residue of the image. Falsehood or error always resides in the added opinion \in the case of something which awaits\ testimony for or against it but in the event receives neither supporting testimony \nor opposing testimony\.6