Some of the different conceptions the term 'substance' has been used to stand for:
A substance is an object or animal or plant, as opposed to whatever is dependent
on such, eg properties. For example, the property of being red is 'dependent'
on the thing that is so coloured.
(Of course you will want to know what 'dependent' means
) This use of 'substance'
starts off in Aristotle, but you will have found it in Descartes too.
A substance in this sense might be defined as whatever is truly independent
of anything else.
Christian theology would say that God is the only substance in this sense.
Descartes agrees with this point of theology, but sometimes uses 'substance' to mean whatever is dependent only on God.
The issue of independence connects with a question that sometimes seems to make sense to some people: What is fundamentally real? Or (is this the same?) What are the fundamental units out of which the Universe is made? So 'substance' is seems sometimes to be used as a term for 'what is fundamentally real', although to understand this at all clearly we would have to get clear about what 'fundamentally' and 'real' mean here.)
(Aristotle had a second sort of substance too. For him, 'second' substances were sorts of things.
You would be specifying a second substance whenever you answered the question What is x? Eg if you replied 'it's a horse' you would be saying that horse was the relevant second substance.
So a crude attempt to catch what Aristotle might have been suggesting would be to say that there are a lot of substances in the world, and that they fall into a number of different types, or second substances.)
If you think of what you are aware of when you perceive a substance in the Aristotelian sense of an object, animal or plant, you are likely to conclude (as a Modern person) that what you are actually aware of are the thing's properties. This generates the question: is there actually any more to an object (or animal or plant) than the set of properties we can experience when we 'perceive it'?
Two responses to this question:
An individual thing on this account consists of a kind of 'substratum' in which properties 'inhere'. The 'substratum' of course has no properties in itself.
It is not clear who if anyone has adopted this conception of 'substance', but Berkeley accuses Locke of holding it.
2.21 There isn't anything more than the properties, but nevertheless when we think of a set of properties as constituting an object we think of that set of properties as 'belonging together' or 'going around together'.
Locke does seem to develop this view, whether to espouse it or reject it or qualify it is uncertain. As Locke presents it, we add to the ideas of the properties that 'go around together' the idea that they all belong together. This then makes up our complex idea of an object (or animal or plant etc.).
Locke calls the idea we add - the idea that the properties all 'belong together' the idea of - wait for it - 'substance'.
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