From the Beginning to Plato

FROM THE BEGINNING TO PLATO 331
Even if sensible objects sustain the same features for some time, they
eventually perish and so could not be what eternal truths are about. If the triangle
is a three-sided plane figure even after a particular sensible triangle perishes, the
general truth cannot be reporting any fact about it. After its demise, no state of
affairs involving the particular triangle exists, so no such state of affairs can be
the reality represented by the general truth.
So, I take Plato to have reasoned, (2) is about a Form, an eternal and
changeless entity, and that is why (2) is an eternal and changeless truth.
Similarly, there will be a Form of Virtue underlying a true definition such as (1),
thereby justifying Plato’s belief in an objective moral reality which is as
independent of human capacities and interests as mathematical reality.
A related and absolutely fundamental point for understanding why Plato
believed in the Forms lies in their role as objects of thought. Its importance is
stressed at the end of the criticism of the Theory of Forms in the Parmenides
when, in face of all the alleged problems for the theory, Plato comes down to one
bedrock argument that furnishes unanswerable proof for the existence of Forms:
If one does not allow Forms of things in view of all the present difficulties
and others like them, and does not distinguish some single Form in each
case, one will have nothing on which to fix one’s thought, since one is not
allowing that in each case there is an Idea that is always the same, and so
one will utterly remove the possibility of discourse.
(135b–c)
Plato sees thought as involving an awareness of entities external to the thinker
where these entities furnish the contents of the thought. For, first of all, when I
think of triangularity I am thinking about something, my thought has a content.
So, Plato (fallaciously) reasons, what I am thinking of exists. Therefore
triangularity is a being that I am aware of when thinking of triangularity. And
second, to think of triangularity is not to be aware of some thought inside my
own mind. Thought is directed toward a content other than itself—a Form
(Parmenides 132b–c).
So thought, like perception, mentally connects us with a reality outside
ourselves, and Plato regularly speaks of thinking as a kind of mental vision. By
thinking of triangularity I stand in a relation to a being which is the content of the
thought. And since I can think of triangularity when no particular triangles exist,
particular triangles could not be the reality I am then related to and aware of in
thinking of triangularity. For what is thought when I think of triangularity does
not vary with the shifting population of particular triangles. So at no time could
the object of thought be identified with sensible objects. 3
For Plato, this shows not merely that the object of thought—the Form of
triangularity—differs from particular triangles, it proves the Form’s complete
independence of them. Hence, Forms are not only eternal and changeless, they
exist independently of what happens in the sensible world.