From The Republic:
“The trait of which I am speaking, I replied, may be also seen in the dog, and is remarkable in the animal.
Why, a dog, whenever he sees a stranger, is angry: when acquaintance, he welcomes him, although the one how never done any harm, nor the other any good. Did this never strike you as curious?
The matter never struck me before; but I quite recognize the truth of your remark.
And surely this instinct of the dog is very charming; — your dog is a true philosopher.
Why, because he distinguishes the face of a friend and of an enemy only by the criterion of knowing and not knowing. And must not an animal be a lover of learning who determines what he likes and dislikes by the test of knowledge and ignorance?
And is not the love of learning the love of wisdom, which is philosophy?”