The study of philosophies should make our own ideas flexible. We are all of us apt to take certain general ideas for granted, and call them common sense. We should learn that other people have held quite different ideas, and that our own have started as very original guesses of philosophers. A scientist is apt to think that all the problems of philosophy will ultimately be solved by science. I think this is true for a great many of the questions on which philosophers still argue. For example, Plato thought that when we saw something, one ray of light came to it from the sun, and another from our eyes and that seeing was something like feeling with a stick. We now know that the light comes from the sun, and is reflected into our eyes. We don' t know in much detail how the changes in our eyes give rise to sensation. But there is every reason to think that as we learn more about the physiology of the brain, we shall do so, and that the great philosophical problems about knowledge are going to be pretty fully cleared up. But if our descendants know the answers to these questions and others that perplex ns today, there will still be one field of which they do not know, namely the future. However exact our science, we cannot know it ns we know the past. Philosophy may be described as argument about things of which we are ignorant. And where science gives us a hope of knowledge it is often reasonable to suspend judgment. That is one reason why Marx and Engels quite rightly wrote to many philosophical problems that interested their contemporaries. But we have got to prepare for the future, and we cannot do so rationally without some philosophy. Some people say we have only got to do the duties revealed in the past and laid down by religion, and god will look after the future. Other say that the world is a machine and the course of future events is certain, whatever efforts we may make. Marxists say that the future depends on ourselves, even though we are part of the historical process. This philosophical view certainly does inspire people to very great achievements. Whether it is true or not, it is powerful guide to action. We need a philosophy, then, to help us to tackle the future. Agnosticism easily becomes an excuse for laziness and conservatism. Whether we adopt Marxism or any other philosophy, we cannot understand it with-out knowing something of how it developed. That is why knowledge of the history of philosophy is important to Marxists, even during the present critical days. What is the main idea of this passage?