Now the search for a monistic explanation of things is a universal tendency of human thought. Wherever we look in the world of thought, we find that this monistic tendency appears. I have already said that it appears throughout the history of philosophy. It reveals itself, {64} too, very clearly in the history of religion. Religion begins in polytheism, the belief in many gods. From that it passes on to monotheism, the belief in one God, who is the sole author and creator of the universe. In Hindu thought we find the same thing. Hindu thought is based upon the principle that "All is one." Everything in the world is derived from one ultimate being, Brahman. But not only is this monistic tendency traceable in religion and philosophy; it is also traceable in science. The progress of scientific explanation is essentially a progress towards monism. In the first place, the explanation of isolated facts consists always in assigning causes for them. Suppose there is a strange noise in your room at night. You say it is explained when you find that it is due to the falling of a book or the scuttling of a rat across the floor. The noise is thus explained by assigning a cause for it. But this simply means that you have robbed it of its isolated and exceptional position, and reduced it to the position of an example of a general law. When the water freezes in your jug, you say that the cause of this is the cold. It is an example of the law that whenever the cold reaches a certain degree, then, other things being equal, water solidifies. But to assign causes in this way is not really to explain anything. It does not give any reason for an event happening. You cannot see any reason why water should solidify in the cold. It merely tells us that the event is not exceptional, but is an example of what always happens. It reduces the isolated event to a case of a general law, which "explains," not merely this one event, but possibly millions of events. It is not merely that cold solidifies the water in your jug. {65} It equally solidifies the water in everybody's jug. The same law "explains" all these, and likewise "explains" icebergs and the polar caps on the earth and the planet Mars. In fact scientific explanation means the reduction of millions of facts to one principle. But science does not stop here. It seeks further to explain the laws themselves, and its method is to reduce the many laws to one higher and more general law. A familiar example of this is the explanation of Kepler's laws of the planetary motions. Kepler laid down three such laws. The first was that planets move in elliptical orbits with the sun in one focus. The second was that planets describe equal areas in equal times. The third was a rather more complicated law. Kepler knew these laws from observation, but he could not explain them. They were explained by Newton's discovery of the law of gravitation. Newton proved that Kepler's three laws could be mathematically deduced from the law of