The question I wish to raise is this. Is it correct to say that one of Plato’s achievements in this passage is the ‘discovery of the copula’, or the ‘recognition of ambiguity of ἔστιν’ as used on the one hand in statements of identity and on the other hand in attributive statements? I feel little doubt that it is correct to say this, but Cornford and Robinson (to mention no others) deny it. After a remark on the question itself I shall try state briefly a case for answering it affirmatively, and shall then consider some of the counter-arguments that have been put forward.
Ackrill, John L., "Plato's Sophist 251-259" (1955). The Society for Ancient Greek Philosophy Newsletter. 233.