In my dissertation I examine Plato's ontology. As is well-known, Plato accepts the existence of two different kinds of entities: sensible particulars, such as Socrates, and Forms, such as the Form of Beauty. Moreover, according to the standard understanding of Plato, he thinks that there are entities of no other kind. I argue that this is an incorrect understanding. Specifically, I argue that Plato assumes the existence of entities of a third kind in the Phaedo. ;The dissertation has developed from (...) my attempts to clarify Plato's conception of the human soul. By examining the Phaedo, the dialogue in which Plato gives his most elaborate discussions of the ontological status of the soul while attempting to prove its immortality, I conclude that Plato conceives the soul as an entity whose ontological status is intermediate between those of Forms and sensible particulars: It has some of the features used to characterize forms and some of the contrary features used to characterize sensible particulars. ;In the dissertation, I first examine the features characterizing forms and those characterizing sensible particulars. Second, I discuss the first group of intermediate entities, the opposites in us, and argue that they are intermediate in the sense in question. Third, I discuss the group of entities that I call the occupiers, such as fire or the number three, and show that they are also seen as intermediates. Finally, I discuss the human soul as an intermediate entity belonging to this second group of intermediates, and show that the Final Argument for the immortality of the soul in the Phaedo can be best understood with reference to the framework including the intermediates as well as forms and sensible particulars. (shrink)