Updating this old thread.
I happen to have just read the passage where Lucretius explains about the soul and it being composed of both the spirit anima and the mind animus.
Spirit, encompassing sensation, as feedback from the world, obtained (having its seat) through the whole body and Mind, encompassing reason and emotion, being felt (the seat, again) in the chest.
I don't know and can't remember whether he (Lucretius) stated that the soul is the thing that inextricably correlates with animals being alive, that is interacting, and reasoning and feeling about this interaction, with the world (as opposed to plants that interact by means of sensation only, or rocks that don't do either), but that's the way I understand it, at this point in my journey anyway.
Putting it less poetically, it seems to me that the soul is, among other things, but possibly primarily, our nervous system.
Would love to read other's take on this.