Aristotle, Nicomachean Ethics
H. Rackham, Ed.

But the facts do not accord with these theories; nor is this surprising. For we admit that one should love one's best friend most; but the best friend is he that, when he wishes a person's good, wishes it for that person's own sake, even though nobody will ever know of it. Now this condition is most fully realized in a man's regard for himself, as indeed are all the other attributes that make up the definition of a friend; for it has been said already¹ that all the feelings that constitute friendship for others are an extension of regard for self. Moreover, all the proverbs agree with this; for example, ‘Friends have one soul between them,’² ‘Friends' goods are common property,’ ‘Amity is equality,’ ‘The knee is nearer than the shin.’³ All of these sayings will apply most fully to oneself; for a man is his own best friend. Therefore he ought to love himself most.

So it is naturally debated which of these two views we ought to adopt, since each of them has some plausibility.