By St. John of Damascus
Philosophy is knowledge of things which are in so far as they are, that is, a knowledge of the nature of things which have being.
And again, philosophy is knowledge of both divine and human things, that is to say, of things both visible and invisible.
Philosophy, again, is a study of death, whether this be voluntary or natural. For life is of two kinds, there being the natural life by which we live and the voluntary one by which we cling lovingly to this present life. Death, also, is of two kinds: the one being natural, which is the separation of soul from body, whereas the other is the voluntary one by which we disdain this present life and aspire to that which is to come.
Still again, philosophy is the making of one's self like God. Now, we become like God in wisdom, which is to say, in the true knowledge of good; and in justice, which is a fairness in judgment without respect to persons; and in holiness, which is to say, in goodness, which is superior to justice, being that by which we do good to them that wrong us.
Philosophy is the art of arts and sciences of sciences. This is because philosophy is the principle of every art, since through it every art and science has been invented. Now, according to some, art is what errs in some people and science what errs in no one, whereas philosophy alone does not err. According to others, art is that which is done with the hands, whereas science is any art that is practiced by the reason, such as grammar, rhetoric, and the like.
Philosophy, again, is a love of wisdom. But, true wisdom is God. Therefore, the love of God, this is the true philosophy.
(From The Fountain of Knowledge: "Philosophical Chapters", Ch. 3)