384
THE POPULAR SCIENCE MONTHLY.
the popular mode of apprehension is distinguished from the scientific in being a compound of experience and metaphysics, and we have here such an instance. But his own clear and guarded statement is as follows:
Nor does the doctrine of the persistence of energy support the idea of a causal relation between the mental and material. "The doctrine of the persistence of energy is a purely physical doctrine. Such an extension would imply the possibility of finding a common measure for the mental and material. Now, what denominator is common to a thought and a material movement, or what common form serves for both? Until such a common form can be pointed out, all talk about an interaction between the mental and material is, from a scientific point of view, unjustified. So long as we confine ourselves to the material we are on safe ground, and so long as we confine ourselves to the mental we are on safe ground; but any attempt to represent a transition from physical to psychological laws, or conversely, brings us face to face with the inconceivable. The causal concept can not be employed to connect two factors which have no common measure. Again: