This perpetual change, then, is what makes conscious life. It is only by virtue of this endless procession of fleeting phases of consciousness that the human soul exists at all. It is thus that we are made. Why we should have been made thus is a question aiming so far beyond our ken that it is idle to ask it. We might as well inquire whether Infinite Power could have made twice two equal five. We must rest content with knowing that it is thus we were created; it is thus that the human soul exists. Just as dynamic astronomy rests upon the law of gravitation, just as physics is based upon the properties of waves, so the modern science of mind has been built upon the fundamental truth that consciousness exists only by virtue of unceasing change. Our conscious life is a stream of varying psychical states which quickly follow one another in a perpetual shimmer, with never an instant of rest. The elementary psychical states, indeed, lie below consciousness, or, as we say, they are sub-conscious. We may call these primitive pulsations the psychical molecules out of which are compounded the feelings and thoughts that well up into the full stream of consciousness. Just as in chemistry we explain the qualitative differences among things as due to diversities of arrangement among compounded molecules and atoms, so in psychology we have come to see that thoughts and feelings in all their endless variety are diversely compounded of sub-conscious psychical molecules. Musical sounds furnish us with a simple and familiar illustration of this. When the sounds of taps or blows impinge upon the ear slowly, at the rate of not more than sixteen in a second, they are cognized as separate and non-musical noises. When they pass beyond that rate of speed, they are cognized as a continuous musical tone of very low pitch; a state of consciousness which seems simple, but which we now see is really compound. As the speed of the blows increases, further qualitative differences arise; the musical tone rises in pitch until it becomes too acute for the ear to cognize, and thus vanishes from consciousness. But this is far from being the whole story; for the series of blows or pulsations make not only a single vivid fundamental tone, but also a multifarious companion group of fainter overtones, and the diverse blending of these faint harmonics constitutes the whole difference in tone quality between the piano and the flute, the violin and the trumpet, or any other instruments. If you take up a violin and sound the F one octave above the treble staff, there are produced, in the course of a single second, several thousand psychical states which together make up the sensation of pitch, fifty-five times as many psychical states which together make up the sensation of tone quality, and an immense number of other psychical states which together make up the sensation of intensity. These psychical states are not, in any