The problem set at the close of our first chapter was to find a fit explanation of the guiding power apparent in natural phenomena. We have not been able to interpret this guiding power either in terms of conscious, intelligent contrivance or in terms of blind, mechanical law. The investigations which followed have led us up to another explanation. We have seen that the vivifying, transforming, progressive power in nature may be conceived as a power of Response. Every particle of matter, organic and inorganic, has this power. Every particle of matter can react and respond to some stimulus. The more it can respond to, the higher it is in the scale of being. And we have found, as I think, one constant and universal stimulus to which both the fixity of nature’s laws and the plasticity of her mysterious substance may be conceived as a response. This stimulus is the call of Life. Stimulus and response taken together constitute the directive force in obedience to which the world unfolds itself in the evolutionary process. We have been led to interpret nature as the concrete expression of the will to live, a will which for the first time comes into rational consciousness in man. Having brought this conception, I hope, into clear light, it is the aim of the present chapter to illustrate and enforce it in more detail, and thus to gain a secure foundation for the application of the conception to the more strictly human problems with which we have ultimately to deal. It must be confessed that the existence in nature of any directive power transcending and utilizing the mechanical forces and relations of matter, call it ‘vital force,’ the ‘hand of Providence,’ the ‘_X_’ of evolution, or what one will, has never readily been admitted by scientific naturalists. They feel that, if once admitted, it offers a prompt and facile explanation of every difficulty, and is available as the cheap resource of all those who study nature with a view to the grinding of their moral or religious axes, rather than to the discovery of truth. Those who feel obliged to believe in the existence of some such power are therefore bound to be more than ordinarily on their guard against all loose thinking. They must not be content with vague generalities, but must be prepared to indicate as exactly as possible the distinction existing between the mechanical and the non-mechanical or transcendental agencies in nature. It does not follow that one’s account of the matter will prove to be exactly true in every detail. One must always speak in such matters with that wise reservation of Socrates, “If this be not the truth, something of the kind is.” But it is not allowable to fall back on that “something of the kind” until an attempt has been made definitely to establish the “kind,” by searching into the inmost heart of the fact.