Rachel asked:

What is the soul made of?

Answer by Helier Robinson

Traditionally the soul is a substance, in Aristotle’s sense of the word: that is, a substance is that which is always a subject, never a predicate. In a proposition such as ‘Socrates is mortal’ Socrates, as a person, is a subject and mortality is a predicate. But Socrates is never a predicate, hence is a substance. Aristotle said that a human being consists of two substances, a body and a soul. He did this to get around the problem (widely ignored today) that, logically, one thing cannot both change with time and also remain one: if it changes then the earlier and the later are necessarily two.

Aristotle’s approach seems to cure the problem, but in fact does not: Aristotle thought that the soul gave a person his/her identity (oneness) by never changing, while the body changed. Theologians liked this because if the soul is unchanging then it must be immortal, thus proving life after death. A second point is that if a philosopher finds it necessary to talk about things which are real but which cannot be perceived, then he has to make some basic assumption about this imperceptible reality. The assumption is always that this reality is rational, in two ways: it is rational in that it is consistent — it contains no contradictions — and it is rational in that it contains necessities corresponding to the logical necessities in the theory, and these real, imperceptible necessities are causal necessities.

However there are two senses of ‘rational’: logic, and mathematics. Plato chose mathematical rationality, but his student Aristotle did not like mathematics and was, besides, the inventor of logic. It was a subject-predicate logic. That is, all logical inferences were between categorical propositions, which are propositions composed of a subject and a predicate of that subject. Aristotle’s famous example of a valid argument is ‘All men are mortal, Socrates is a man, therefore Socrates is mortal.’ So for Aristotle the imperceptible reality consisted of substances (corresponding to subjects) and attributes (corresponding to predicates). Speculation concerning imperceptible reality was, of course, called metaphysics, and most contemporary philosophers will have nothing to with it, on the Popperian grounds that it is in principle unfalsifiable.

But that does not stop investigation into the nature of imperceptible reality; only now it is called theoretical science. (Theoretical means non-empirical, which means imperceptible.) As an activity it is far more successful than metaphysics, primarily because it is grounded on empirical science. However, it also differs from metaphysics in that the rationality assumed for it is not Aristotelean logic, but mathematics. And mathematics has no concepts of substances and attributes. Instead, if you want to confine it to one concept, the subject matter of mathematics is structure. Consequently in modern science there is no such thing as a soul; the concept of souls is as obsolete as the concepts of phlogiston and the electromagnetic ether. You can, if you wish, distinguish mind (consciousness) from body, but mind is an structure emergent out of brain, and as such can only be immortal if the brain is immortal.