How do sensible objects, such as people, interact with Forms? Socrates provides a discussion of a simple (or perhaps, not-so-simple) case. Simmias, with whom Socrates is speaking, is taller than Socrates but shorter than Phaedo. In some sense, then, Simmias is both tall and short. But Tallness itself (the Form) can never be short, nor can Shortness itself ever be tall. The Forms, after all, if they really exist, must be perfect exemplars of the property they represent. So the tallness “in us” will never be short, but when, for instance, Phaedo approaches Simmias, one of two things must happen: either the tallness in Simmias flees, or it perishes. Meanwhile, however, Simmias is still Simmias because he is not defined by his tallness or shortness.[15] So it appears that the property of being tall comes to and goes from Simmias according to who he stands next to.
At this point, an anonymous member of the party objects, saying that earlier Socrates had maintained that opposites came to be from opposites, but now we are saying that opposites never change into their opposites. Socrates replies, “You have boldly reminded us of something, but you don’t understand the difference between the point we are making now and the one we were making then. Then we were saying that one thing comes from its opposite; now, however, we are saying that the Opposite itself will never become opposite to itself—neither the opposite in us nor the one in nature. Then, my friend, we were talking about the things having opposites, the things getting their names from the opposites; now, however, we are talking about the very things which, when they are present, give their names to the objects. It is these latter things that we say never admit of coming-to-be from each other.”[16]
Here Socrates points us back to a discussion early in the dialogue when he said that things come from their opposites.[17] Plato here is reminding us that sensible objects that are the subjects of changes change from opposite to opposite, for instance, from ugly beautiful, hot to cold. Now we are talking about the opposites such as Beauty and Ugliness, Hot and Cold. The Forms can never change their natures, but sensible objects can change from instantiating one property to instantiating its opposite. Plato’s student Aristotle will always focus on the role of the subject of the change, while Plato always focuses on the character exemplified by the subject. Indeed, Plato has no regular terminology for the objects of the sensible world that allegedly instantiate properties. For him, they are always in some important sense, dependent on the properties they have for their existence, whereas for Aristotle the properties will depend on the “substance” (ousia) or concrete object which instantiates them. Plato sees the ultimate realities of the universe as the Forms which ephemeral objects instantiate by “participating in” those Forms.
[15] Plato Phaedo 102b-e.
[16] Plato Phaedo 103b-c.
[17] Plato Phaedo 70d-71a.