It is an article of the common faith of mankind that consciousness is good and trustworthy evidence of the reality of that of which we are conscious. It is also characteristic of that common faith to believe in the trustworthiness of the Power which manifests itself in that of which we are conscious. The man of science shares in the common faith of mankind up to a certain point: he accepts the testimony of consciousness to the reality of material things, and he believes that the Power which manifests itself in them can be trusted to behave when it is (in time or space) beyond the range of his observation in exactly the same way as it does within. But to walk in the common faith further than this point is unscientific. It is rational to trust the evidence of consciousness when it testifies to the reality of material things, but not when it testifies to the reality of our moral ideals, or the freedom of the will or the reality of God. It is scientific to trust the Power which manifests itself in consciousness to behave with the same uniformity in the future as it has done in the past, and rational to formulate our science and stake our material interests on that uniformity. But it is not rational or scientific to trust that Power to will freely the good of all things, or to trust our lives to that will. The reason of this sharp division between science and faith is the mistaken idea that science involves no faith and is a body of knowledge built up without any assumption. But even if we got the man of science to admit that science would be impossible if things were not real and Nature not uniform, it would still be open to him to say that he considered any other assumptions unnecessary; and there is a way in which he could prove them to be unnecessary. He might show that they were no assumptions at all, but logical consequences from established scientific facts. That was in effect the object aimed at, as far as our moral ideals are concerned, by the optimistic philosophy of evolution. For the optimistic philosopher, then, who refuses to begin by taking the difference between right and wrong on faith, the problem is, granted the reality of material things and the uniformity of Nature, to show that the moral law is simply one particular case of the uniformity of Nature.