The Republic by Plato Part 2 Page 61

I do not apprehend your meaning.

The trait of which I am speaking, I replied, may be also seen in the dog, and is remarkable in the animal.

What trait?

Why, a dog, whenever he sees a stranger, is angry; when an acquaintance, he welcomes him, although the one has never done him any harm, nor the other any good. Did this never strike you as curious?

The matter never struck me before; but I quite recognise the truth of your remark.

And surely this instinct of the dog is very charming; — your dog is a true philosopher.

Why?

Why, because he distinguishes the face of a friend and of an enemy only by the criterion of