Q (Martin): Was Plato a monist or a dualist?
I am not so sure. At the top of the pyramid of knowledge, the summum bonum or supreme Good reigns by Itself: it is the supreme archetype (arché) or nous. Plato’s is a scalar ontology, the lower steps or hypostases being subservient to the higher ones (like the 5 koshas of Vedanta and the 5 levels in Sufism). Contemplation only can afford such view. I think that would make Plato a monist – or non-dualist.
A (Tom McFarlane): Despite Plato’s Theory of Forms, which is widely viewed today as a form of dualism, there is good reason to believe that Plato was not a dualist and did not view his Theory of Forms as his final position.
In fact, none other than Plato himself demolished the Theory of Forms in his Parmenides dialogue. In the first half of that dialogue, he has the character of Parmenides point out to the young Socrates all the problems and contradictions that result from such a dualistic theory.
Parmenides concludes by saying: These difficulties and many more besides are inevitably involved in the forms, if these characters of things really exist and one is going to distinguish each form as a thing just by itself. The result is that the hearer is perplexed and inclined either to question their existence or to contend that, if they do exist, they must certainly be unknowable by our human nature.
Plato was thus completely cognizant of the problems with the theory of forms, or at least a naive understanding of it. Consequently, it is implausible that Plato adhered to that doctrine himself, i.e., he did not in the end maintain a dualism involving a second world of forms apart from the sensible world of particular things. It is more plausible that Plato had a much more subtle view of the forms, a view in which the realms of being and becoming do not separate forms from sensible objects; rather, being and becoming are both implicit in the nature of the forms. As Plato wrote: By Heaven, can we be ready to believe that the absolutely real has no share in movement, life, soul, or wisdom? That it does not live or think, but in solemn holiness, unpossessed of mind, stands entirely at rest? That would be a dreadful thing to admit.
So, according to Plato at least, the sensible objects we experience are not separate things apart from some inaccessible world of intelligible forms, but they are instantiations of the forms themselves. In other words, to the extent that our experience is intelligible at all, we are directly experiencing forms, which ultimately emanate from the One. This understanding of Plato is also reflected in the Platonic tradition by Proclus and Plotinus, for example.