A very small amount of thinking is enough to show that what I call my knowledge of the external world, is merely a knowledge of my own mental modifications. A step further and I find that my idea of causation as a principle in the external world is derived from my knowledge of this principle in the internal world. For I find that my idea of force and energy in the external world is a mere projection of the idea which I have of effort within the region of my own consciousness; and therefore my only idea of causation is that which is originally derived from the experience which I have of this principle as obtaining among my own mental modifications. If once we see plainly that the idea of causation is derived from within, and that what we call the evidence of physical causation is really the evidence of mental modifications following one another in a definite sequence, we shall then clearly see, not merely that we have no evidence, but that we _can have_ no evidence of causation as proceeding from object to subject. However cogent the evidence may appear at first sight to be, it is found to vanish like a cloud as soon as it is exposed to the light of adequate contemplation. In the very act of thinking the evidence, we are virtually denying its possibility as evidence; for as evidence it appeals only to the mind, and since the mind can only know its own sequences, the evidence must be presenting to the mind an account of its own modifications; from the mere fact, therefore, of its being accepted as thinkable, the evidence is proved to be illusory.