The truth must be admitted that thought works under strange conditions. Pure 'reason' is only one out of a thousand possibilities in the thinking of each of us. Who can count all the silly fancies, the grotesque suppositions, the utterly irrelevant reflections he makes in the course of a day? Who can swear that his prejudices and irrational opinions constitute a less bulky part of his mental furniture than his clarified beliefs? And yet, the _mode of genesis_ of the worthy and the worthless in our thinking seems the same. =The laws are cerebral laws.= _There seem to be mechanical conditions on which thought depends, and which_, to say the least, _determine the order in which, the objects for her comparisons and selections are presented_. It is a suggestive fact that Locke, and many more recent Continental psychologists, have found themselves obliged to invoke a mechanical process to account for the _aberrations_ of thought, the obstructive prepossessions, the frustrations of reason. This they found in the law of habit, or what we now call association by contiguity. But it never occurred to these writers that a process which could go the length of actually producing some ideas and sequences in the mind might safely be trusted to produce others too; and that those habitual associations which further thought may also come from the same mechanical source as those which hinder it. Hartley accordingly suggested habit as a sufficient explanation of the sequence of our thoughts, and in so doing planted himself squarely upon the properly _causal_ aspect of the problem, and sought to treat both rational and irrational associations from a single point of view. How does a man come, after having the thought of A, to have the thought of B the next moment? or how does he come to think A and B always together? These were the phenomena which Hartley undertook to explain by cerebral physiology. I believe that he was, in essential respects, on the right track, and I propose simply to revise his conclusions by the aid of distinctions which he did not make. =Objects are associated, not ideas.= We shall avoid confusion if we consistently speak as if _association_, so far as the word stands for an _effect, were between_ THINGS THOUGHT OF--_as if it were_ THINGS, _not ideas, which are associated in the mind_. We shall talk of the association of _objects_, not of the association of _ideas_. And so far as association stands for a _cause_, it is between _processes in the brain_--it is these which, by being associated in certain ways, determine what successive objects shall be thought.