sensations and images and their relations. Beliefs, desires, volitions, and so on, appeared

to us to be complex phenomena consisting of
sensations and images variously interrelated. Thus (apart from certain relations) the occurrences which seem most distinctively mental, and furthest removed from physics, are, like physical objects, constructed or inferred, not part
of the original stock of data in the perfected science.
>From both ends, therefore, the difference between physical and psychological data is diminished. Is there ultimately no difference, or do images remain
as irreducibly and exclusively psychological? In view of the causal definition of the difference between images and sensations, this brings us to a
new question, namely: Are the causal laws of psychology different from those of any other science, or are
they really physiological? Certain ambiguities must be removed before this question can be adequately discussed. First, there is the
distinction
between rough approximate laws and such as appear to be precise and

general. I shall return to the former presently; it is the latter that I wish to discuss now. Matter, as defined at the end of Lecture V, is
a logical fiction, invented because it gives a con