On this living trunk of nature our own life is grafted; animals come into existence, and among animals the human species. Each of us, as he comes into the world, finds this nature, developed, abundant, diversified in millions of forms, traversed by innumerable forces, organised up to the most highly developed structures, man included. We find this nature, and we begin to study it. We examine its parts one by one, their complexity, and the difference of their functioning. For each one of them has its peculiar way of being and of acting; it has its "laws." The aggregate of these laws, mutually corresponding, and integrating one another, constitutes the natural world--reality--as it stands before us. With this external reality we strive to become acquainted; and in order that we may live in it we either adapt ourselves to it, or adapt its conditions to ourselves. In this reality too we acquire the knowledge of the needs of our organism and of the means by which they may be satisfied,--the ratio, so to speak, between natural desires and controlled resources. We are also told that our organism is in constant change and hurries on to its destination, to our death, which we abhor as passionately as we cherish life, but which we accept because such is the law of human life, fatal and inexorable; for reality is what it is, and we must adapt ourselves to it. But if reality appears as constituted before us, as therefore conditioning our existence, and as existing independently of us; if it is indifferent to reality whether we be in it or not; if we are truly extraneous to it, the conclusion must then be drawn that we, from the outside, presume to know reality and to move about it without being this reality itself or any part of it. For all reality is thought by us as a connected whole, though indeed vaguely; in its totality it is regarded as an object known to us, but existing in utter independence of this knowledge of ours. Its whole process is therefore complete in objective nature, which conditions our spiritual life, and this in turn can mirror reality but can never be a part of it. This then is the primitive and fundamental concept that the human mind forms of reality. In consequence of it man feels that he is enclosed within himself: he knows he is producing the dreams and the fair images of art; that he can construct inwardly abstract geometrical figures and numbers; that he can generate ideas. But he also feels that between these ideal creations of his own, and the solid, sound, real living forms of nature, there is an abyss. He must, indeed, fall in with nature, in the process of generating other living beings of flesh and blood. He must avail himself of nature by first submitting to its unfailing laws, if he intends to give body, that is, real existence, to the ideal conceptions of his intelligence. On one side then we have thought; on the opposite side reality,--that reality, Nature.