Full Idea
You do not, Socrates, have an inkling of how great the difficulty is if you are going to posit one form in each case every time you make a distinction among things.
Gist of Idea
Forms are very difficult, if we must posit a new Form every time we make a distinction
Source
Plato (Parmenides [c.366 BCE], 133b)
A Reaction
For example, is there a platonic form for each of the 39 isotopes of tin? Is there a new form each time humans create a new artifact, such as model 16 of the Apple iPhone? A rather good objection, from fictional Parmenides.
Book Reference
Plato: 'Parmenides', ed/tr. Gill,M.L./Ryan,P. [Hackett 1996], p.135