The phenomena of nature (using that term in its widest sense) are not only a series of sequences, they are also the revelation of a mysterious Power or living Force. All that we perceive by the senses, and, inductively register in nature, is a series of phenomena, of which the laws of nature are the generalized expression and interpretation. But every change is a revelation not only of succession, but of causal power. No matter where we take our stand along the line of sequence, mental or material, always and at every point this conviction is flashed in upon the mind, 'there is a hidden Power behind.' But we instinctively ask, 'what is this power or force determining the changes of the universe?' Is it material or spiritual? Can the force which moves the particles of matter be material? We do not perceive it by the senses, which take note only of the modified phenomena of matter. It is neither visible, nor audible, nor tangible. It is invisible; must we not therefore believe it to be incorporeal? We cannot reach it by analysis. We conclude that it is not physical but hyper-physical, not natural but supranatural. We have an intellectual intuition of it. It announces its presence in every change that occurs, but it nowhere shows its face as a material entity. It is a mystic agency endlessly revealing its existence, everywhere concealing its source. We watch its evolutions, but it escapes our scrutiny; we try to detain it, and we find that it is gone; yet it reappears in the next thing we examine, and in the very phenomena of our search for it; the agency is manifest, but it is the Agent we wish to discover. Must it be, like the sangreal of mediæval legend, sought for in many lands, but nowhere found by any wanderer in quest of it?