Aristotle, Presaging the Cogito and Categorical Imperative
The Niomachean Ethics of Aristotle, trans. Robert William Browne (George Bell and Sons: 1889), pp. 252-4.
Now he that sees, perceives that he sees; and he that hears, that he hears; and he that walks, that he walks; and in every other case, in the same manner, there is some faculty which perceives that we are energizing; so that we perceive that we are perceiving, and understand that we are understanding. But this is the same as saying that we perceive or understand that we exist; for existence was defined to be perceiving, or understanding. Now, to perceive that one is alive, is of the number of those things which are pleasant in themselves: for life is a good by nature: and to perceive the good which is inherent in one's self is pleasant. But life is eligible, and particularly to the good, because existence is to them good and pleasant; for by the consciousness of that which is absolutely a good, they are pleased.
Now, the good man has the same relation to his friend as he has to himself; for a friend is another self; in the same manner, therefore, as to exist one's self is eligible to every one, so also is it for one's friend to exist, or nearly so. But existence was said to be eligible on account of the perception of that which is a good: and such a perception is pleasant in itself. We ought, therefore, to be conscious of the existence of our friend; and this would result from associating with him, and sharing his words and thoughts; for this would seem to be the meaning of the word society, when applied to men, and not, as in the case of cattle, the merely feeding in the existence. If, then, existence is in itself eligible to the happy man, being by nature something good and pleasant, and if the existence of a friend is nearly the same, then a friend must also be of the number of eligible things. But that which is eligible to a man, he ought to possess; or else he is deficient in that respect; he, therefore, that is to be happy will need good friends.