In his An Essay Concerning Human Understanding, Locke declares that rational parrots “have passed for a race of rational animals,” but they are still parrots and not human beings, “for I presume it is not the idea of a thinking or rational being alone that makes the idea of a man in most people’s sense: but of a body, so and so shaped, joined to it: and if that be the idea of a man, the same successive body not shifted all at once, must, as well as the same immaterial spirit, go to the making of the same man.”
Further, Locke says: “Since I think I may be confident, that, whoever should see a creature of his own shape or make, though it had no more reason all its life than a cat or a parrot, would call him still a man; or whoever should hear a cat or a parrot discourse, reason, and philosophize, would call or think it nothing but a cat or a parrot; and say, the one was a dull irrational man, and the other a very intelligent rational parrot. A relation we have in an author of great note, is sufficient to countenance the supposition of a rational parrot."
Gottfried Wilhelm Leibniz is in agreement with Locke on this issue. In his New Essays on Human Understanding, Leibniz says, “there is no obstacle to there being rational animals of some other species than ours… Indeed it does seem that the definition of ‘man’ as ‘rational animal’ needs to be amplified by something about the shape and anatomy of the body; otherwise, according to my views, Spirits would also be men.”
In an earlier paragraph, Leibniz says: “I think I may be confident that anyone who saw a creature with a human shape and anatomy would call it ‘a man’, even if throughout its life it gave no more appearance of reason than a cat or a parrot does; and that anyone who heard a parrot talk and reason and philosophize wouldn’t describe it or think of it as anything but a parrot. We would all say that the first of these animals was a dull irrational man, and the second a very intelligent rational parrot.”