A scientific view of something is always an intimate mixture of theories and observed facts. The theories are broad, general ideas together with arguments based on them. The arguments are designed to show that, if the general ideas are accepted, then this or the other thing ought to be observed. If this, that, or the other actually is observed, then the theory is a good one; if not, then the theoreticians have to think again. Thus, theoretical ideas and arguments are continually subjected to the severe test of comparison with the facts, and scientists are proud of the strictness with which this is done. On the other hand, theories often suggest new things to look for; in other words, they lead to predictions. These predictions are frequently successful, and scientists are entitled to be proud of that, too. But it follows that no theory is immutable; any scientific view of any subject may, in principle, be invalidated at any time by the discovery of new facts.