This paper reconstructs the account of concept formation developed in the 4th Century A.D. by Themistius in the most ancient extant commentary on Aristotle’s Posterior Analytics. Themistius’ account can be contrasted with two widespread modern interpretations of Aristotle. Unlike psychological empiricists, Themistius ascribes an active role in concept formation to our innate capacity of understanding. Unlike intuitionists, he would not be satisfied by saying that νοῦς “intuits” or “spots” concepts. Rather, the question is what makes our νοῦς capable of “finding” and “recognizing” concepts in experience, and this can only be an understanding prior to all experience. Themistius seems to be responding here to Platonist arguments against Aristotle’s epistemology: postulating a “potential νοῦς” is not enough, for one can apply Meno’s dilemma to it and ask how it can recognize that it has found what it was looking for. But, contrary to the judgment of some modern scholars, Themistius never embraced the theory of recollection either. He argued that both empiricism and Platonist innatism are wrong and developed a middle path marked by a strong interdependence between the perceptive and the rational capacity. This holds for all rational learning, and concept formation is its first stage: to form a concept means to learn something genuinely new, but also to recognize it as falling, e. g., under one of the ten categories. While being presented as a mere “paraphrasis” of Aristotle’s words, Themistius’ account is a well-advised and original response to the epistemological debates of his time.