the.com/theory of forms

means Plato's idea that beyond the physical things we see and touch, there exist perfect, unchanging, non-physical 'Forms' (like Beauty, Justice, or Circularity itself) of which all actual objects are just imperfect, temporary copies.

from Rooted in Greek 'eidos' or 'idea,' meaning 'that which is seen' or 'shape' — from the verb 'idein,' to see. Plato took a word for visible appearance and flipped it: the truest 'form' of a thing, he argued, is precisely what you cannot see with your eyes, only grasp with the mind. The doctrine is laid out across dialogues like the Republic and Phaedo, most famously dramatized in the Allegory of the Cave, where prisoners mistake flickering shadows on a wall for reality itself.