We must first consider thought quite impartially without relation to a thinking subject or to an object of thought. For subject and object are both concepts constructed by thought. There is no denying that thought must be understood before anything else can be understood. Whoever denies this, fails to realise that man is not the first link in the chain of creation but the last. Hence, in order to explain the world by means of concepts, we cannot start from the elements of existence which came first in time, but we must begin with those which are nearest and most intimately connected with us. We cannot, with a leap, transport ourselves to the beginning of the world, in order to begin our analysis there, but we must start from the present and see whether we cannot advance from the later to the earlier. As long as Geology fabled fantastic revolutions to account for the present state of the earth, it groped in darkness. It was only when it began to study the processes at present at work on the earth, and from these to argue back to the past, that it gained a firm foundation. As long as Philosophy assumes all sorts of principles, such as atom, motion, matter, will, the unconscious, it will hang in the air. The philosopher can reach his goal only if he adopts that which is last in time as first in his theory. This absolutely last in the world-process is thought. There are people who say it is impossible to ascertain with certainty whether thought is right or wrong, and that, so far, our starting-point is a doubtful one. It would be just as intelligent to raise doubts as to whether a tree is in itself right or wrong. Thought is a fact, and it is meaningless to speak of the truth or falsity of a fact. I can, at most, be in doubt as to whether thought is rightly employed, just as I can doubt whether a certain tree supplies wood adapted to the making of this or that useful object. It is just the purpose of this book to show how far the application of thought to the world is right or wrong. I can understand anyone doubting whether, by means of thought, we can gain any knowledge of the world, but it is unintelligible to me how anyone can doubt that thought in itself is right. ADDITION TO THE REVISED EDITION (1918).