Editing Problem of universals (section)

=== Plato ===
Plato believed that there was a sharp distinction between the world of perceivable objects and the world of universals or [[substantial form|forms]] (eidos): one can only have mere opinions about the former, but one can have [[knowledge]] about the latter. For Plato, it was not possible to have knowledge of anything that could change or was particular, since knowledge had to be forever unfailing and general.<ref>MacLeod & Rubenstein (2006), §1b.</ref> For that reason, the world of the forms is the real world, like [[Allegory of the cave|sunlight]], while the sensible world is only imperfectly or partially real, like [[Allegory of the cave|shadows]]. This [[Platonic realism]], however, in denying that the [[theory of forms|eternal Forms]] are mental artifacts, differs sharply with modern forms of idealism.

One of the first nominalist critiques of Plato's realism was that of [[Diogenes of Sinope]], who said "I've seen Plato's cups and table, but not his cupness and tableness."<ref>{{cite book|last=Davenport|first=Guy|others=Translated by Guy Davenport|title=Herakleitos and Diogenes|publisher=Grey Fox Press|location=Bolinas|year=1979|pages=[https://archive.org/details/herakleitosdioge00hera/page/57 57]|isbn=0-912516-35-6|url=https://archive.org/details/herakleitosdioge00hera/page/57}}</ref>