CHAPTER V. THE EMANCIPATION OF SCIENTIFIC RESEARCH FROM THE YOKE OF PHILOSOPHICAL THEORIES. The excessive use of Hypothetical Agents in Physiological Explanations—§ 1. _Vital Phenomena in Fully-constituted Organisms_—Provisory Exclusion of the Morphogenic idea—The Realm of the Morphogenic Idea as the Sanctuary of Vital Force—§ 2. _The Physiological Domain properly so called_—Harmony and Connection of Phenomena—Directive Forces—Claude Bernard’s Work—Exclusion of Vital Force, of Final Cause, of the “Caprice” of Living Nature—Determinism—The Comparative Method—Generality of Vital Phenomena—Views of Pasteur. The theories whose history we have just sketched in broad outline long dominated science and exercised their influence on its progress. This domination has ceased to exist. Physiology has emancipated itself from their sway, and this, perhaps, is the most important revolution in the whole history of biology. Animism, vitalism, materialism, have ceased to exercise their tyranny on scientific research. These conceptions have passed from the laboratory to the study; from being physiological, they have become philosophical. This result is the work of the physiologists of sixty years ago. It is also the consequence of the general march of science and of the progress of the scientific spirit, which shows a more and more marked tendency to separate completely the domain of facts from the domain of hypotheses. _Excessive Use of Hypothetical Agents in Physiological Explanations._—It may be said that in the early part of the nineteenth century, in spite of the efforts of a few real experimenters from Harvey to Spallanzani, Hales, Laplace, Lavoisier, and Magendie, the science of the phenomena of life had not followed the progress of the other natural sciences. It remained in the fog of scholasticism. Hypotheses were mingled with facts, and imaginary agents carried out real acts, in inexpressible confusion. The soul (_animism_), the vital force (_vitalism_), and the final cause (_finalism_, _teleology_) served to explain everything.