The theories of evolution which we have called the Optimistic and the Pessimistic interpretations of evolution are avowedly based on the assumption that a large part of the common faith of mankind is a mental or moral disease. According to Mr. Herbert Spencer the faith that we can know what is real is an illusion: the Real is the Unknowable. According to Professor Huxley the common faith in the freedom of the will is an illusion: necessity is the law of the uniformity both of Nature and of human nature. In thus declining to accept the testimony of the moral and religious consciousness as evidence of what is, both philosophers were influenced by the belief that it is science alone which is capable of ascertaining and demonstrating what is and what actually does happen. This belief, however, we have ventured to suggest, overlooks two facts. One is that the abstract sciences do not even profess to state what actually does happen: they simply affirm that, if the conditions stated in their various laws are the only conditions operative, the only result will be that stated by the particular law in question. Thus science does not concern itself with what is or does happen, but solely with what would be or would happen under certain (usually impossible) conditions. The other point overlooked is that the historical or comparative sciences are also only hypothetically true. All that their laws undertake to demonstrate is that, if certain consequences constitute the whole of an observed effect, then the only conditions antecedently operative were those stated in the law. Here too, then, science does not even claim to prove what is or demonstrate what does happen, but assumes that we know it or find it out, in some way with which science does not concern itself. If we do know and can know what is, science can tell us what were the conditions that produced it.