What now is Spencer's theory of an ultimate reality that must for ever remain unknowable? Following a line of thought that had been steadily gaining ground since Hume--although much older than Hume--Spencer holds that in final analysis all our knowledge is a knowledge of mental states and their relations. Beyond this we _know_ nothing, and can never know anything. Nevertheless, while we cannot know anything beyond consciousness, the conditions of thinking oblige us to assume that something exists as the cause of our states of mind. Just as black implies something that is not black, hard something that is not hard, so we must conceive, as against the conditioned, relative existence of our conscious states, an unconditioned, absolute existence as their cause. It is this assumed, but completely unknown cause of our conscious states, and of all else, that Spencer distinguishes as the Unknowable, the Unconditioned, the Absolute, etc., and which appears to have brought so much consolation to hard-pressed theists. I have no intention of discussing here the philosophic value of the "Unknowable." But one may say, in passing, that even from that point of view Spencer is untrue to his own Agnosticism in speaking of the Unconditioned as the _cause_ of phenomena. For causation is a category of the conditioned, it belongs to the world we know. It is not something that exists beyond consciousness, it is something that is supplied by consciousness and which possesses validity only within the world of phenomena. On Spencer's own theory of relativity a cause only exists in relation to an effect. Destroy the one and you destroy the other. Thus, if the Unknowable is a cause of phenomena it ceases to be the unconditioned and becomes part of the phenomenal order. If, on the other hand, it is not part of the phenomenal sequence, it cannot stand to phenomena in a genuine casual relation. It is, however, only fair to point out that between the Unknowable and the evolutionary philosophy of Spencer the only connection between them is that they are both in the same work. In all probability it is an unconscious survival of Spencer's earlier theism, which was active at the time the Synthetic Philosophy was originally planned, but which became more and more attenuated as Spencer grew older, and disappears entirely from the more important volumes of the series. And but for the help it has been supposed to give the belief in god, the "Unknowable" would only have ranked as a harmless speculation of no value to anyone or to anything. This is substantially admitted in a postscript to the 1899 edition of "First Principles." At the conclusion of the section entitled "The Unknowable," he says:--