Schopenhauer on What Constitues Philosophy

In most introductory philosophical textbooks, the standard definition of philosophy presented is usually etymological in nature. It is pointed out that philo and sophy are derived from the words that mean love and wisdom respectively in Ancient Greek, so philosophy is presented as an enquiry into wisdom and left at that.

For the purposes of an introductory textbook, such a definition I suppose is decent enough. Schopenhauer, however, gets much closer to the truth, if not hitting it directly, when he talks of philosophy as follows in his The World as Will and Representation:

The mode of explanation employed in various fields of study explains things in reference to one another, but it always leaves unexplained something that it presupposes. In mathematics, for example, this is space and time; in mechanics, physics, and chemistry, it is matter, qualities, original forces, laws of nature; in botany and zoology, it is the difference of species and life itself; in history, it is the human race with all its characteristics of thought and will. And in all these it is the principle of sufficient reason in the form appropriate to each. Philosophy has the peculiarity of presupposing absolutely nothing as known; everything to it is equally strange and a problem, not only the relations of phenomena, but also those phenomena themselves.