For only after men had tried their hand for thousands of years at merely objective philosophizing did they discover that, among the many things that make the world so puzzling and precarious, the first and foremost is that, however immeasurable and massive it may be, its existence hangs nevertheless on a single thread; and this thread is the actual consciousness in which it exists.

--Arthur Schopenhauer, The World as Will and Representation, Vol. II

Indeed, it might be said that without further help everyone knows what the world is, for he himself is the subject of knowing of which the world is representation, and so far this would be true. But this knowledge is a knowledge of perception, is in the concrete. The task of philosophy is to reproduce this in the abstract, to raise to a permanent rational knowledge sucessive, variable perceptions, and generally all that the wide concept of feeling embraces and describes merely negatively as not abstract, distinct, rational knowledge. Accordingly it must be a statement in the abstract of the nature of the whole world, of the whole as well as of the parts. However, in order not to be lost in an endless multitude of particular judgements, it must make use of abstraction, and think everything individual in the universal, and its differences also in the universal. It will therefore partly separate, partly unite, in order to present to rational knowledge the whole manifold of the world in general, according to its nature, condensed and summarized into a few abstract concepts. Yet through these concepts, in which it fixes the nature of the world, the whole individual as well as the universal must be known, and hence the knowledge of both must be closely bound up. Therefore, aptitude in philosophy consists precisely in what Plato put it in, namely in knowing the one in the many, and the many in the one.

--Schopenhauer, Will and Representation 1:15