“I speak of what I scarcely understand; but the soul when thinking appears to me to be just talking—asking questions of herself and answering them, affirming and denying.”
Socrates describes thought as a conversation conducted silently within oneself. He is trying out an account of judgment while discussing false opinion with Theaetetus, and even qualifies the description with an admission of uncertainty. It is a dialogue hypothesis about thinking, not a confirmed psychological theory.
Source details
Plato, Theaetetus, approximately 189e–190a; Socrates responding to Theaetetus on what it means for the mind to conceive something. Benjamin Jowett English, Project Gutenberg #1726, main dialogue after the discussion of false belief. The same passage later distinguishes inner speech from externally spoken words.