Forms are Plato’s substances, for everything derives its existence from Forms. In this sense of ‘substance’ any realist philosophical system acknowledges the existence of substances. Probably the only theories which do not would be those forms of logical positivism or pragmatism... The second use of the concept is more specific. According to this, substances are a particular kind of basic entity, and some philosophical theories acknowledge them and others do not... According to Plato, the governing principles were the intelligible Forms which material objects attempted to copy. These Forms are not substances in the sense of being either the stuff or the individuals or the kinds of individuals out of which all else is constructed. Rather they are the driving principles which give structure and purpose to everything else.

The same article lists 6 basic qualities that 'substance' is usually taken as having (and a 7th for Kant). This seems to be from Aristotle (though "the pre-socratic philosophers in fact had a concept of substance rather like that above attributed to chemistry"), who showed "the marks and characteristics of a primitive concept on which we have an intuitive grasp".

It seems Plotinus' "neo-platonism" was intended as a restatement of Plato's views, and that Plotinus thought there were two kinds of material substances (sensible and intelligible substance, with the One or Good, had by all things, not being a substance (From Plato to Platonism, unknown page)).

The relation between Plato and Aristotle is commonly portrayed as a contrast between a philosophy of essence and a philosophy of substance, but Ricoeur shows that this opposition is too simple. Aristotelian ontology is not a simple antithesis to Platonism: the radical ontology of Aristotle stands in a far more subtle relation of continuity and opposition to that of Plato

What would Plato have said about 'substance' in the "second use of the concept"?


One might disagree with the interpretation of ousia as substance and claim that its meaning is better grasped as essence. Adopting the latin essence one might claim further that essence is over and beyond being. Anyway understanding proceeds through binary distinctions: essence vs substance or form vs substance and dualistic thinking acknowledges this logic while monisms try somehow to evade it. Taking Plato to be a paradigmatic monist one has to look what stays in contrast to his pure forms and a good place for this is his Timaeus. Plato adopts the 4 elements but takes them to be (the so called platonic) solids and he reduces them to 2 types of triangles which are the ultimate constituents of everything. Triangles are indisputably forms but we would say that they are spatial forms, which is the meaning of the greek word for them (eidos). So the ultimately substance we could discover in Plato is space for which he uses the specific word "khora". (An unserveyable amount of writings discuss it but the article in Cassin's Dictionary of Untranslatables could be a starting point)

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