What is the soul?

My understanding of the "official" description of the soul is that it's comprised of very fine atoms distributed (I think) throughout the body. This is off the top of my head; I believe Lucretius discusses it but I don't have a cite at the moment.

Book 3 of Lucretius has a lengthy discussion of the mind and the soul. Or of course the podcast episodes dealing with Book 3.

Yes I would say that the description in Lucretius Godfrey refers to is the basic material.

If I had to summarize my understanding I would also refer to the "properties of the particles" and "qualities" / "events" of the bodies that are formed from the particles. Probably a large part of what most people are looking for is a discussion of how life arises from non-life, and that issue revolves around the "emerging" aspect of how bodies have qualities that are not inherent in the particles and void of which they are composed.

It also seems that "soul" and "spirit" are used almost interchangeably.

I note in the original post the reference to "all living things" having a soul, and I am not sure about that. Plants? I am thinking that Epicurus would not necessarily equate a soul/spirit to life, and that you might we'll be able to have life without having a soul - that soul / spirit might be why we call some but not all living things "animals".

Also, following the point made in Long's "Chance and Natural Law in Epicureanism" I think it is important to keep in mind that "the swerve" does not exhibit itself to our view except in certain situations (free will of animals primarily). If they indeed thought that way, that would explain the emphasis on the soul being extremely fine / light / fast moving - characteristics in which the swerve might best "break through" to allow free will.

I don't have nearly as good a command of where things are in Lucretius as I should have. I just checked one of my references however and definitely Book 3 is the main text, but there is some material in Book 2, especially near the end, that discusses how the living arises from the non-living, and that is pretty relevant to the topic too.

Here is one of my online topical outlines of Lucretius:

Lucretius - On The Nature of Things []

I think your wording is good. As in other cases it seems to me that Epicureanism is practical. It does not represent to dwell on technical details but addresses big picture questions (what are we?) and answers them with a practical framework. In this case, that which we think of as uniquely "us" is composed of certain types of atoms arranged in certain ways that are entirely natural. That answer doesn't necessarily allow us to have sufficient knowledge to go out and build a human being from scratch, but it gives us an understanding of what to expect from life and excludes worries or fears of supernatural issues beyond our control.

We can then choose to pursue the technical details as much or as little as we are able and as we prefer within a framework that makes sense.

I just found this interesting excerpt, on the Stanford Encyclopedia of Philosophy website, "Ancient Theories of Soul" - Section on Epicurus:

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5.1 Epicurus’ Theory of Soul

Epicurus is an atomist, and in accordance with his atomism he takes the soul, like everything else that there is except for the void, to be ultimately composed of atoms. Our sources are somewhat unclear as to exactly which kinds of materials he took to be involved in the composition of soul. It is very probable, though, that in addition to some relatively familiar materials — such as fire-like and wind-like stuffs, or rather the atoms making up such stuffs — the soul, on Epicurus’ view, also includes, in fact as a key ingredient, atoms of a nameless kind of substance, which is responsible for sense-perception. Thus it seems that while he thought he could explain phenomena such as the heat or warmth of a living organism, as well as its movement and rest, by appealing to relatively familiar materials and their relatively familiar properties, he did feel the need to introduce a mysterious additional kind of substance so as to be able to explain sense-perception, apparently on the grounds that “sense-perception is found in none of the named elements” (L&S 14C). It is worth noting that it is specifically with regard to sense-perception that Epicurus thinks the introduction of a further, nameless kind of substance is called for, rather than, for instance, with regard to intellectual cognition. What this suggests, and what in fact we have independent reason to think, is that on Epicurus’ view, once one is in a position adequately to explain sense-perception, one will then also be in a position to work out an explanation of intellectual cognition, by appropriately extending the explanation of sense-perception. Let us consider briefly how such extension might work.

Perceptual beliefs, like the belief that ‘there is a horse over there’, will be explained, in Epicurus’ theory, in terms of sense-impressions and the application of concepts (‘preconceptions’; for discussion cf. Asmis 1999, 276–83), and concept-formation is in turn explained in terms of sense-impression and memory. According to Diogenes Laertius’ summary (L&S 17E1–2), the Epicureans say that

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preconception is, as it were, cognition or correct belief or conception or universal ‘stored notion’ (i.e. memory), of that which has frequently become evident externally: e.g. ‘such-and-such a kind of thing is a man’. For as soon as the word ‘man’ is uttered, immediately its impression also comes to mind by means of preconception, as a result of antecedent sense-perceptions.

Moreover, sense-impressions, interpreted and articulated in terms of concepts or preconceptions, yield experience concerning evident matters, which in turn forms the basis for conclusions about non-evident matters. For example, extensive experience can make clear to one not only that the human beings one has interacted with have a certain feature (say, rationality), but also (later Epicureans will say, probably somewhat developing Epicurus’ position) that it is inconceivable that any human being could fail to have that feature (cf. L&S 18F4–5). And so, experience will not only make one expect, with a very great deal of confidence, that any human being one will ever encounter anywhere will be rational. Experience also, according to the Epicureans, supports the inference to, and hence justifies one in accepting, the (non-evident) conclusion that all human beings, everywhere and at all times, are rational (for detailed discussion, cf. Allen 2001, 194–241). This obviously is an extremely generous view of what experience, and ultimately sense-perception, can do! Once we recognize the enormously powerful and fundamental role Epicurus and his followers assign to sense-perception, we will not be surprised to see that they feel the need to include in the composition of the soul a very special kind of material that accounts specifically for sense-perception, but apparently do not think that, in addition to that, some further special material is needed to enable intellectual or rational activity.

In the Epicurean tradition the word ‘soul’ is sometimes used in the broad traditional way, as what animates living things (e.g., Diogenes of Oenoanda, fr. 37 Smith), but the focus of interest, so far as the soul is concerned, is very much on the mental functions of cognition, emotion and desire. A view that is common in the tradition and that very probably goes back to the founder is that the soul is a composite of two parts, one rational, the other nonrational. The rational part, which Lucretius calls mind [animus], is the origin of emotion and impulse, and it is also where (no doubt among other operations) concepts are applied and beliefs formed, and where evidence is assessed and inferences are made. The nonrational part of the soul, which in Lucretius is somewhat confusingly called soul [anima], is responsible for receiving sense-impressions, all of which are true according to Epicurus. Error arises at a later stage, when sense-impressions are interpreted by the rational part of the soul, in a way that, as we have seen, crucially involves memory. Sense-perception, conceived of simply as the reception of sense-impressions by the nonrational soul, does not involve memory (cf. L&S 16B1). Since the formation and application of concepts requires memory, sense-perception, so conceived of, does not involve conceptualization, either. The nonrational part is also responsible for transmitting impulses originating from the rational part, as well as (presumably) for a wide variety of other vital functions. (When Epicurus distinguishes between pleasures and pains of the soul and those of the body, incidentally, the distinction he has in mind must be between the rational part of the soul on the one hand and the body animated by nonrational soul, on the other.)

Source: https://plato.stanford.edu/entries/ancient-soul/#5.1

Some good stuff there and it concerns exactly what we are discussing in Episode 246 of the podcast which is of such importance.

"What this suggests, and what in fact we have independent reason to think, is that on Epicurus’ view, once one is in a position adequately to explain sense-perception, one will then also be in a position to work out an explanation of intellectual cognition, by appropriately extending the explanation of sense-perception."

I think that's probably right, and again it points to the central role of reasoning about the IMperceptible based on what IS perceptible. That's how we conclude atoms exist, and no doubt it relates to how Epicurus concluded "gods" exist, but it also answers the charges that skeptics like Cotta raised as to why an Epicurean (if he indeed bases all reasoning on sense experience) would not rule out the possibility of existence of oceans if he had never seen one in the past.

The references cited in the article may be helpful in summing up the answers to that charge, and will allow us to make more clear why the single term "empiricist" does not fully describe Epicurus' method of thinking.