Although Plato's emphasis on rationality and calculative reason, along with his faith in the soul's immortality, place him well to mark a clear-cut distinction between the human and the beast, that clear distinction is somewhat difficult to locate. While there is no strong evidence that Plato was inclined to attribute human-like cognitive capabilities to animals, the absence of extended textual engagements with animal cognition make this an uncertain issue. And in the moral domain, there do seem to be texts which suggest that a kind of natural virtue may be found among animals. These appear in the Laws. and are counterbalanced by earlier denials of those same virtues to the non-human soul. But most interestingly, Plato's descriptions of the inner dynamics and characteristic moral failures of the human soul bring humans and beasts into affinity. Plato's emphasis on the precariousness of the genuinely human condition, on its artifactual nature and the necessity of constant vigilance lest the beast within break its bonds and run to supremacy, have the implication that the clear distinction between human and beast which the Cratvlus etymology signals is itself a result, an achievement, and an ideal. It is in effect a difficult process, rather than a naturally given condition. For Plato, human nature must be won, and won again.
Browning (Cole), Eve, "Plato on the Souls of Beasts" (1991). The Society for Ancient Greek Philosophy Newsletter. 447.