refer it, either at rest or in movement. But it is otherwise if we draw out of the movement the mobility which is its essence. When my eyes give me the sensation of a movement, this sensation is a reality, and something is effectually going on, whether it be that an object is changing its place before my eyes or that my eyes are moving before the object. A fortiori am I assured of the reality of the movement when I produce it after having willed to produce it, and my muscular sense brings me the consciousness of it. That is to say, I grasp the reality of movement when it appears to me, within me, as a change of state or of quality. But then how should it be otherwise when I perceive changes of quality in things? Sound differs absolutely from silence, as also one sound from another sound. Between light and darkness, between colours, between shades, the difference is absolute. The passage from one to another is also an absolutely real phenomenon. I hold then the two ends of the chain, muscular sensations within me, the sensible qualities of matter without me, and neither in the one case nor in the other do I see movement, if there be movement, as a mere relation: it is an absolute. Now, between these two extremities lie the movements of external bodies, properly so called. How are we to distinguish here between real and apparent movement? Of what object, externally perceived, can it be said that it moves, of what other that it remains motionless? To put
