Theory of Forms
Read this description of Plato's theory of the forms. What does Plato mean by "forms"?; How does this relate to justice? What does the ideal state have to do with justice?
The words, εἶδος (eidos) and ἰδέα (idea) come from the Indo-European root *weyd- or *weid- "see" (cognate with Sanskrit vétti). Eidos (though not idea) is already attested in texts of the Homeric era, the earliest Greek literature. This transliteration and the translation tradition of German and Latin lead to the expression "theory of Ideas". The word is however not the English "idea," which is a mental concept only.
The theory of matter and form (today's hylomorphism) started with Plato and possibly germinal in some of the presocratic writings. The forms were considered as being "in" something else, which Plato called nature (physis). The latter seemed as carved "wood", ὕλη (hyle) in Greek, corresponding to materia in Latin, from which the English word "matter" is derived, shaped by receiving (or exchanging) forms.