Many concerns and disputes cluster around the ideas associated with this term. The substance of a thing may be:

1. (i) its essence, or that which makes it what it is. This will ensure that the substance of a thing is that which remains through change in its properties. In Aristotle (Metaphysics Z, vii) this essence becomes more than just the matter, but a unity of matter and form.

2. (ii) That which can exist by itself, or does not need a subject for existence, in the way that properties need objects; hence

3. (iii) that which bears properties.

A substance is then the subject of predication, that about which things are said as opposed to the things said about it. Substance in the last two senses stands opposed to modifications such as quantity, quality, relations, etc. It is hard to keep this set of ideas distinct from the doubtful notion of a substratum, something distinct from any of its properties, and hence incapable of characterization. The notion of substance tends to disappear in empiricist thought in favour of the sensible qualities of things, with the notion of that in which they inhere giving way to an empirical notion of their regular concurrence. But this in turn is problematic, since it only makes sense to talk of the concurrence of instances of qualities, not of qualities themselves. So the problem of what it is for a quality to be instanced remains.

Metaphysics inspired by modern science tends to reject the concept of substance in favour of concepts such as that of a field or a process, each of which may seem to provide a better example of a fundamental physical category.