Up until a few days ago, if you’d asked me who was responsible for the view that “Man is rational animal”, or, more precisely, that rationality (or perhaps risiblity, or perhaps both) was the difference via which the species humans is separated from all other members of the genus animal, I would’ve said “Aristotle, of course. Everybody knows that.”
And if I had said that, I should’ve known better, because what Everybody Knows often actually Nobody Knows, at least if you think knowledge entails truth. I’m currently working on a paper with a friend on concepts of rationality in AI and in people, specifically looking at whether rationality is a normative or a descriptive concept, and whether it differs (or should differ) in the two contexts. When the two of us were discussing the roots of the view that rationality is a normative concept when applied to people, I mentioned casually that surely this view owed at least some debt to Aristotle’s definition of man as rational animal. When the time came to start turning this into an argument, the first thing I did was try to find a citation for this definition, and this is when my suspicions began to rise.
Now, maybe this is not news to anyone reading this blog, that Aristotle didn’t actually define man in this way. I’ll be the first to admit I have not read as much of Aristotle as I should, nor as read it as often. Still, the view is pervasive, and is repeated throughout the internet (always without citation). The closest I found was a discussion of human virtue in the Nichomachean Ethics, framed in terms of the soul. Of the human soul, Aristotle points out “that one element in the soul is irrational and one has a rational principle” (Bk. I, Ch. 13). While the irrational part of the soul is “common to all species and not specifically human” (ibid.), Aristotle does not explicitly say there that the rational part of the soul is not common to all species and is specifically human. The closest we come to that, that I could find, is in the Metaphysics, where men are distinguished from other animals by living not only with the aid of appearances and memory but also “by art and judgement” (τέχνῃ καὶ λογισμοῖς) (Bk. A, ch. I).
So, where, then, does this definition come from? I can’t say for sure where the earliest account of this can be found (any ancient philosophers reading this? Do you know the answer?), but it goes back to at least Porphyry, who uses rationality or reason as a differentia of humanity. Porphyry distinguishes three ways in which things can differ: (1) commonly, (2) properly, and (3) most properly. One object differs from another in the most proper way when the first is distinguished from the other by means of a specific difference, and the example that Porphyry gives is “as a man differs from a horse by a specific difference, that of rational” (ch. 3). Such a difference is ‘most proper’ because, unlike common and merely proper differences, most proper differences make two things not only “otherlike” but actually “other”. As Porphyry puts it, “when the difference rational approaches animal, it makes it other and makes a species of animal\dots it is in virtue of those differences which make a thing other that divisions of genera into species are made” (ibid.). So then, for Porphyry, not only is rationality what distinguishes the species humanity from all the other items in the genus ‘animal’, it is only the species humanity that is characterised by this difference: Any animal which does not possess rationality is other than man.
The moral of the story: It’s always good to be reminded that not everything that Everybody Knows is actually true, and that one should always, always cite their sources, even when — especially when — reporting things that Everybody Knows.