 Timaeus’ opening question (“What is that which always is and never becomes…?”) can be read extensionally (“What entity or entities are such that they always are and never become…?”) or intensionally (“What is it for some entity always to be and never to become…?”). If read in the former way, the answer will be “forms” or “a form.” If read in the latter way, the question is answered immediately in the text  and hence this reading seems preferable : always to be is to be intelligible and unchanging.