Of the origin of energy we have not solved the riddle, we have given it up; but instead of coming from without we know that it comes from within. As Mr Benn puts it, we have extended the atomistic method from “matter” to motion. Harvey’s contemporary, Francis Bacon, sagaciously guessed that heat is an expansive motion of particles; but he regarded heat and cold as two contrary principles. Almost in the same generation the brilliant John Mayow perceived a substance in the air “allied to saltpetre,” which passed in and out of the blood by the way of the lungs or placenta. “Innate heat” then gave way to phlogiston; but it was not till the discovery of oxygen and of the conservation of energy that we attained a theory of energy, and finally got rid of “matter and form,” and of all the thicket of metaphysics, relating thereto; through which in the day of Harvey no mind, however mighty, could have made its way. In the history of medieval thought we must always bear in mind that in neither of its two periods were theology, logic, metaphysics, psychology, or even physics, fully differentiated; and before the Arabian literature they were not differentiated at all[34]. Logic, which for us is but a drill, and, like all drills, a little out of fashion, was for the Middle Ages a means of discovery, nay, the very source of truth; thus every man carried his own busy laboratory within him. The heirs of Porphyry and Boetius had no other method in their possession. The dialectically irresistible was the true (κατάληψις); thus was man to succeed “irrefutabile aperire secretum.” To begin to think before beginning to learn is a hollow business, yet then logic furnished the theorems which experience might illustrate at its leisure; and nature was contemplated under philosophy. The differentiation of psychology began with the translation of the _De anima_[35], and the recognition of the relation of the percipient; hence, in the second period, Roger Bacon denounced the pretensions of logic, and John Duns, that brilliant backslider, forced them to an absurdity. Again, on the translation of the Metaphysics, theology parted into the studies of the doctrines of God and the soul, which belong to theology proper, and of being, in modes, kinds and universals, which belong to metaphysics. Medicine again was a confusion of spheres, as was theology; the care of the soul and the care of the body were the ends of knowledge, and their means contained all knowledge. Thus when we hear that Alcuin ordered the formal teaching of medicine, it was under the name of “Physica”; and not until the Physics of Aristotle came to light did the various branches of natural history become in their turn not only definite studies but also self-sufficient, aside from the art of healing. To this day the healer keeps the name of “physician”; and the subject at Cambridge the name of Physic. It is well to be reminded that although the soldiers of truth