The overall aim of this PhD dissertation is to consider and examine the relations between Forms in Plato’s theory of Forms. Undertaking this task does not require a full account of Plato’s theory of Forms, rather it requires a systematic and comprehensive investigation of how, according to Plato in different dialogues and different stages of his philosophical career, each Form is related to other Forms. For the purpose of this dissertation, I treat as familiar Plato’s claim that sensible things are what they are in relation to and by participating in particular Forms.

The main thesis of the dissertation is that Plato always, both before and after the Parmenides, in which he appears to submit the theory of Forms to critical investigation and assessment, held a single coherent view about the relations between Forms. This is the view that each Form, in addition to being what it is in virtue of itself (e.g. the Form of justice being, in virtue of itself and being the very thing it is, a certain harmony of the soul) has a number of properties in virtue of its relation to certain other Forms. According to this account of Plato’s view, which it is my aim to defend, each Form has a determinate structure, which consists of a combination of what it is in virtue of itself (kath’ auto) and what it is in virtue of its relation to certain other Forms (pros allo).

This overall thesis of the dissertation is of particular interest and significance, not least because it goes against a dominant (though not universally accepted) view in the literature, according to which Plato changed his mind radically about this issue and question in and when he wrote the Parmenides. According to this widely held view among critics, which it is my aim to argue against and refute and replace with a positive account, Plato, before the Parmenides and in such major dialogues as Republic, Phaedo and Symposium, held that individual Forms are what they are simply in virtue of themselves, and that Forms do not stand in relations to other Forms. Likewise according to this widely held view of Plato’s Forms and the theory of Forms, Plato came to realise in the Parmenides that each Form must stand in relations to other Forms, and he developed this view further in such dialogues as the Sophist.

A major aim of this dissertation is to argue against this dominant view in the literature and to show that, on the contrary, Plato holds, throughout these dialogues (including Charmides, Phaedo, Republic, Symposium, Parmenides and Sophist) a single coherent view about Forms, what they are in virtue of themselves, and what they are in virtue of their relations to each other.