The Existence of Forms - Argument from Perfection

[NowAgnostic]

One of Plato’s arguments for the existence of “Forms” (the equivalent to Aristotle’s “universals,” often known today as “abstracta”) is the so-called “argument from perfection.”

I neither agree nor disagree with this. I think it’s something that can neither be proven nor disproven from first principles, and something one choose to accept or not.

We might start by observing a line drawn on a piece of paper. The line is not perfectly straight, but how do we know it’s not perfectly straight unless we already know what a perfectly straight line looks like?

And how do we know it’s not infinitesimally thin unless we know what an infinitesimally thin line looks like? But we can’t see something infinitesimally thin.

The same type of question can be asked about circles and any shape, and indeed, many other things. Let’s put this in the form of a reductio ad absurdum:

Prove A: Forms exist.
Assume ~A: Forms do not exist.
~A → B: If Forms do not exist, then we cannot know that a shape is imperfect.
~B: We can know that a shape is imperfect.
~~A: by modus tollens.
Therefore, A: Forms exist.
Q.E.D.

I am going to take issue with “We can know that a shape is imperfect.” What is the definition of an imperfect shape? If the definition is one that doesn’t correspond exactly with one of the Forms, then the argument is circular - and besides, there might be a Form corresponding to every possible shape, making every shape perfect. True, “circles” and “ellipses” have precise mathematical definitions, but there is no such thing as an imperfect circle or imperfect ellipse - and for shapes you might think fit those categories, I can generate a new mathematical definition which fits them “perfectly”.

 

[stenlis]

The key distinction between these two that Plato would point out is that while unicorns do not exist necessarily, certain Forms arguably do. For example, even if no humans existed, it would still be true that the circumference of a circle is equal to pi times its diameter.

So, while unicorns do not have necessary existence, certain Forms (or universals, etc.) do, at least according to anyone sympathetic to capital-R Realism.

Well, that is an entirely different claim than the one made in the OP. The OP argument is fallacious - thing A does not exist just because you can evaluate whether something real is or is not thing A.

If you want to make a new argument (like you did in the quoted post), start a new thread and we can discuss that one.