(To be sure, this is simply one way to characterize philosophy; there are many others.)
If we think about the very first philosopher, we can understand his novel and bold statements as offering an explanation. We are confronted with the world - the phenomena of the five senses, of daily experience. What causes them? From what did they arise? Historian Anthony Esolen writes:
The first Greeks to call themselves philosophers strove to understand the physical world, to see what prime element underlay clouds and lions and marble and blood. We should not take for granted their bold assumption that such an element could be found, and that the world was intelligible! Thales of Miletus reasoned that such an element must be capable of assuming the three phases of matter: solid, liquid, and gas. Hence he posited that water was somehow the arche or foundation or origin of all things, though he knew well that you couldn't squeeze water to make iron or clay. His successor Anaximenes voted for air. Others named earth or fire or some combination of the four so-called elements.
Thales was exploring the discipline we now call cosmology. What is the systematic structure of the universe? Although we may laugh at his notion that water is the foundational element of the universe, he had rational, if not correct, evidence: it covers 75% of the earth's surface, humans are approximately 75% water, it's necessary for all known forms of life; it's found everywhere on planet earth; etc. Not a bad hypothesis. But another Greek philosopher soon found the problem with the explanation offered by Thales, and the with the explanations offered by many of the other pre-Socratic philosophers:
But there's a logical problem with all explanations of the world that resolve it into such stuff as water or air. To say that the arche of the world is water doesn't explain anything, since water itself is one of the things that requires explaining. It is circular reasoning. Nor does it help to stretch the circle as wide as the cosmos. The philosopher Anaximander, therefore, reason that whatever the arche is, it cannot be like the things it explains. It must be beyond predication. So he called it apeiron or the boundless.
The explanation for the physical world cannot be, according to Anaximander's reasoning, simply one of the ingredients of that world. Later philosophers would use words like 'essence' and 'substance' to try to capture similar ideas. That thing, upon which the physical universe is contingent, cannot be merely one part of the physical universe. It must be something other. Anaximander may get credit for invented that branch of philosophy we now call metaphysics.
