Read "Of Those Sentiments Concerning Nature with Which Philosophers Were Delighted" by Plutarch online for free on Textopian. Full text with search, annotations, highlights, and AI-powered reading aids.
An idea is a being incorporeal, which has no subsistence by itself, but gives figure and form unto shapeless matter, and becomes the cause of its manifestation.
Socrates and Plato conjecture that these ideas are essences separate from matter, having their existence in the understanding and fancy of the Deity, that is, of mind.