 Although one could say that the bodies that surround our bodies … are in some sense the cause of the ideas we then have … because these are material substances, the action of which does not extend to the soul insofar as it is simply a thing which thinks … they cannot be more than their remote and occasional causes which, by the union of mind and body, cause our faculty to think and determines it to produce ideas of which the faculty of thinking itself is the principal and effective cause. (T 176; THM 92)