But the discussion must not end here. Hitherto I have presented the case Liberty _versus_ Necessity with all the impartiality of which I am capable; but I have done so without travelling an inch beyond those limits of discussion within which the question has been debated by previous writers. I believe, indeed, that I have pointed out several important oversights which have been made on both sides of the question; but in doing this I have not gone further than the philosophical basis upon which the question has been hitherto argued. My object, however, in publishing these papers is not that of destructive criticism; and what I have done in this direction has been done only in order to prepare the way for what is now to follow. Having shown, as it appears to me conclusively, that upon both the rival theories of Materialism and Spiritualism--the doctrine of Liberty, and therefore of Moral Responsibility--must logically fall, I now hope to show that this doctrine admits of being re-established on a basis furnished by the theory of Monism. It often happens that an elaborate structure of argument, which is perfectly sound and complete upon the basis furnished by a given hypothesis, admits of being wholly disintegrated when the fundamental hypothesis is shown to be either provisional or untrue. And such, I believe, is the case with the issue now before us. For the issue Liberty _versus_ Necessity has hitherto been argued on the common assumption that natural causation is not merely the most ultimate principle which the human mind can reach; but also a principle which is, in some way or another, external to that mind. It has been taken for granted by both sides in the controversy that if our volitions can be proved to depend upon natural causation, as rigid in its sequences within the sphere of a human mind as within that of a calculating machine, there must be an end of the controversy; seeing that our volitions would be thus proved to be rigidly determined by those same principles of fixed order, or 'natural law,' which are external to, or independent of, the human mind--quite as much as they are external to, or independent of, the calculating machine. Now, it is this assumption which I challenge. The theory of Monism entitles one to deny that when we have driven the question down to the granite bed of natural causation, nothing more remains to be done; according to this theory it still remains to be asked, What is the nature of this natural causation? Is it indeed the ultimate datum of experience, below which the human mind cannot go? And is it indeed so far external to, or independent of, the human mind, that the latter stands to it in the relation of a slave to a master--coerced as to action by the conditions which that master has laid down?