"If it is your desire," he wrote, "to discourse fluently and learnedly about philosophical questions, begin with the Ionians and work steadily through to the latest new speculative treatise. If you have a good memory and a fair knowledge of Greek, Latin, French, and German, three or four years spent in this way should enable you to attain your object. If, on the contrary, you are animated by the much rarer desire for real knowledge; if you want to get a clear conception of the deepest problems set before the intellect of man, there is no need, so far as I can see, for you to go beyond the limits of the English tongue. Indeed, if you are pressed for time, three English authors will suffice, namely, Berkeley, Hume, and Hobbes." The first and perhaps the greatest problem in metaphysics can be put very shortly. What is the reality behind the apparent universe of matter and mind we see around us? Or, rather, what do we know of that reality? To the uninitiated in philosophical thinking it seems sufficiently plain that there are two entities, body and soul in man, matter and mind in the whole universe; and various types of intelligent dogmatists, ranging from the sturdy if somewhat stupid shrewdness of Dr. Johnson to the agile casuistry of Catholic metaphysicians, have supported this simple verdict of "common sense." Trouble begins, however, with any attempt to analyse the relations between what we call "matter" and what we call "mind." It appears, for instance, that what we call matter we only know in terms of mind. In an essay on Descartes's _Discourse on Method_, Huxley explains this by simple examples.