Lucretius supposed that mind and soul (in Latin, animus and anima) are parts of the body, and because all bodies, including the bodies of animals, are material things, they also are material things. They are organs, which is to say they do certain sorts of work that are more or less essential to the body as a living thing, which is suitably called an organism. Hands are organs, so is the tongue, the heart. And so are mind and soul.
Lucretius believed that mind and soul perform the most essential work of a living body. The soul, which is distributed throughout the body, is the principle of life or vitality, and of earthly spirituality, which is emotion infused throughout the body, sometimes calming, sometimes exciting, the only spirituality that is.
When the soul ceases to function, the body dies. The mind is the instrument of understanding and will, and it coordinates its operations with the soul, which also has other functions that don’t normally depend on thinking and willing, such as respiration. Their functions are much like those we now attribute to the brain and the central nervous system.
The account of mind and soul given in Book III of De Rerum Natura is crude but in its essentials notably similar to contemporary scientific theory, in particular, to neuroscience. Lucretius imagined that mind and soul consist of the finest of atoms, which was fitting if the body was to account for the quickness of thought and action, for according to the atomic theory he imagined matter not as some gross and coarse stuff, but as active, vital, subtle and mostly invisible. So, although mind and soul are material, they were not crude lumbering things, but fine and capable.
Whereas soul was spread out to all parts of the body, enabling it to feel and to act, mind was situated in one strategic place. Lucretius, following Epicurus, located the mind in the midsection of the body, rather than in the head. This was because operations of mind included in his judgment not only clear refined thoughts, but also profound emotions and “gut feelings,” which find expression in thoughts and discourse.
The theory of soul and mind fleshed out in De Rerum Natura is an empirical one, based on observation and reasoning. It is not the product of the haunted reflections of a philosopher, alone in his room, like the celebrated René Descartes, whose only discovery was the very tenuous one of his own momentary ghostly existence.
Here again, the preferred method is of a naturalist, featuring a keen observation of animals, mostly human ones, at all stages of life. The method is suited to Lucretius’ purpose, which is to persuade his readers that mind and soul are robustly physical, yet like the body and all of its parts, they are also mortal.
The evidences that Lucretius gathers in support of this conclusion are, even after two millennia, just as relevant and persuasive and are no less worth pondering. He observes that, from infancy to old age, the mind and the soul’s vitality go through cycles of growth and decay, just like all other parts of the body, like all animate things, and this implies that they are mortal like the rest.
Moreover, like other parts of the body, the mind is subject to “foul diseases and harsh pain,” to depression, fear, grief, madness, which, like other diseases of the body, may cause death or at least loss of self-identity. Often, when the body is diseased, mind and soul follow suit, suffering dementia, delirium and extreme lethargy.
And should the patient die, are we to suppose that soul and mind do not die also, but are set free into the air? If so, then what is their state? With what means do they travel? Have they senses and other organs, and if not, how do they recognize things or communicate with them? Besides, the mind has its own diseases that ravage it even when the body seems otherwise perfectly healthy.
So, Lucretius argues, if a healthy body cannot safeguard the welfare and good health of the mind, what would happen to the mind if it were allowed to float off, ghost-like or smoke-like, on its own, unable to withstand the buffeting of the wind, blind and unprotected by the walls of the body, so that it lacks even the sense of touch?
Furthermore, if mind and soul are not parts of the body, how do we explain drunkenness, which affects body, mind and soul? When wine enters the veins, a man staggers, he loses the power of speech, his eyes are unable to focus, his mind grows cloudy, and he loses all control of his body and all sensibility. But material things can affect the mind in a beneficial way also, which also suggests that it is a material thing.
Then and now, it seems, that the mind, “like a sick body, can be healed and changed by medicine,” which restores it to sanity, to rationality and sensibility. The theory behind the practice may have changed, but the practice is essentially the same. How could this be if the mind were not a material thing, if there were not a chemical connection between the mind and the medicines that treat the its maladies? And how could this be, unless the mind were a material thing, an organ?
If mind and soul are organic parts of a mortal body, then they too are mortal. They die when the body dies, and all that remains is given back to nature. Even more important than this, when the body dies the self is no more.
What is the self? It is the consciousness we have in our waking moments, and perhaps in our dreams also, of being the same person, a sense of the ownership of our thoughts, and feelings and actions in the present, the memory of things past, and expectations of what will come. Lucretius observes that even if, after our death, all our physical remains were gathered together, and the body revived consisting of the very same elements, this would not be the same person, not ourselves. Resurrection is a mere fantasy.
Death is like a dreamless sleep from which there is no awakening. It is nothing. Should this trouble us? Is this not a matter of much ado about nothing? Before I was born, the universe had existed for an eternity, and so it will continue after I die. If the former does not trouble me, neither should the latter.
“Therefore death is nothing to us,” or to give Lucretius’ Latin its proper emphasis: “Nothing, therefore, is death to us.”