Early in book 1 of De Anima, Aristotle considers how to characterize the passions, and concludes, “τὰ πάθη λόγοι ἔνυλοί εἰσιν” (403a26). Passions are “reasons embedded in matter.” For example, he suggests that anger can appropriately be depicted a series of bodily movements in response to certain stimuli (“τὸ ὀργίζεσθαι κίνησίς τις τοῦ τοιουδὶ σώματος…ὑπὸ τοῦδε ἕνεκα τοῦδε”) (403a27-8).
But then he doubles back, noting that while natural philosopher (physikos) can describe anger as a physiological matter, he doesn’t ipso facto displace the logician (dialektikos). Both define anger, albeit differently: “ὁ μὲν γὰρ ὄρεξιν ἀντιλυπήσεως ἤ τι τοιοῦτον, ὁ δὲ ζέσιν τοῦ περὶ καρδίαν αἵματος καὶ θερμοῦ. τούτων δὲ ὁ μὲν τὴν ὕλην ἀποδίδωσιν, ὁ δὲ τὸ εἶδος καὶ τὸν λόγον.” (403b1-4). The philosopher will situate anger in the space of reasons, as a “certain desire for vengeance” – viewed this way, anger has a particular intentional character, ordered to a history and to a future. The scientist, by contrast, will view it as a matter of physiology – a disturbance in particular organs (the amygdala, we’d now say). The scientist describes the matter of anger, but the philosopher describes its form and nature (τὸ εἶδος καὶ τὸν λόγον).
Aristotle gives the lie to those who claim that scientific accounts of mind are rivals to the “folk psychology” of classical philosophy — he certainly recognized a place for depicting the physiological states correlated to — and so constituting necessary conditions for — various “passions,” and he surely would’ve been interested in the finer-grained accounts of those states offered by neuroscientists. But he denied outright that the passions, or any other mental state, was reducible to such an account — that would be to leave aside the logos which makes the action intelligible as a human action. The materialist doesn’t explain; he just explains away.