29. A glance at the metaphysiological definitions will detect both the kind of guess and the kind of reliance which prevailed. The mystery was not simply recognized, it was personified as an entity: Will and Intelligence were liberally accorded to it, for it was supposed to shape matter, and direct force into predestined paths by prescience of a distant end. The observed facts of the egg passing through successive changes into a complex organism were so marvellous, so unlike any facts observable in the inorganic world, that they seemed to demand a cause drawn from higher sources. The mystery of life obtruded itself at every turn. It was named, and men fancied it explained. But in truth no mystery is got rid of by explanation, however valid; it is only shifted farther back. Explanation is the resolution of a complex phenomenon into its conditions of existence--the product is reduced to its factors; the explanation is final when this resolution has been so complete that a reconstruction of the product is possible from the factors. The vast majority of explanations--especially in the organic region--are no more than what mathematicians call “a first approximation.” It is through successive approximations that science advances; but even when the final stage is reached a mystery remains. We may know that certain elements combine in certain proportions to produce certain substances; but why they produce these, and not different substances, is no clearer than why muscles contract or organisms die. This Why is, however, an idle question. That alone which truly concerns us is the How, and not the Why. 30. Biology is still a long way off the How. But it can boast of many approximations; and its theories are to be tested by the degree of approximation they effect. In this light the physiological, _intra_-organic, hypotheses manifestly have the advantage. Many of them are indeed very unacceptable; they are guided by a mistaken conception of the truths reached by Analysis. For when men first began to discard the extra-organic hypotheses, and to look into the organism itself, they were so much impressed by the mechanical facts observed, that they endeavored to reduce all the phenomena to Mechanics. The circulation became simply a question of hydraulics. Digestion was explained as trituration. The chemists then appeared, and their shibboleths were “affinities” and “oxidations.” With Bichat arose the anatomical school, which decomposing the organism into organs, the organs into tissues, and these tissues into their elements, sought the analytical conditions of existence of the organism in the properties of these tissues, and the functions of these organs. The extra-organic agent was thus finally shown to be not only a fiction, but a needless fiction.