Full Idea
In Plato's use of the expression 'idea' we can see that he understood by it something that not only could never be borrowed from the senses, but even goes beyond the concepts of the understanding, since nothing in experience could be congruent to it.
Gist of Idea
Plato's Forms not only do not come from the senses, but they are beyond possibility of sensing
Source
report of Plato (The Republic [c.371 BCE]) by Immanuel Kant - Critique of Pure Reason B370
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.395
A Reaction
This is why Kant is not a Platonist - because he thinks the limits of our world are the limits of our capacity for possible experience, and Platonic Forms exceed that limit. Personally I am with Plato. I'll never experience a quark either.