with space and time it suffices to determine the character of the transcendent reality. On the contrary, the conception of a force-center involves a reference to the feeling of effort which accompanies spontaneous motion, and so in describing things-in-themselves as centers of force a realist metaphysic ascribes a transcendent significance to a single kind of sensation, while it yet regards sensation in general as merely subjective. Its agreement with natural science in this point is unavailing; for the distinction between the primary and secondary qualities of things has only methodological validity. Thus the realist metaphysic fails to pass from the sphere of sensible phenomena to the transcendentally real, and the thing-in-itself remains a mere duplicate of the object of perception.
The fact is that transcendental realism has not taken account of the Kantian maxim that concepts without sensible intuition to give them content are empty, and v. Hartmann recognized this, and protests that such concepts cannot exist. Admitting, however, that many of his criticisms of Kant are valid, it yet remains a simple fact that our thinking cannot transcend the data of experience, and that even when the right of thought to work up and elaborate these data has been fully conceded, nothing absolutely beyond experience can ever be reached. All that metaphysic, therefore, can effect is the progressive intellectualizing of the content of experience, transforming objects of perception into objects of thought.
An explanation of nature is a tracing back phenomena to force, shock, or work. This is done by first classifying phenomena under laws, then by grouping laws under theories. The essential and certain portion of a theory is composed of differential equations, among which those fundamental are the ones which bind up with time and space; force, work, and shock. These three notions, derived from the sense of touch and the muscular sense, are at the bottom of all science. Next come the logical principles, contradiction and identity; then axioms of mathematical science. From these we draw algebra and calculus; adding space,—geometry; adding time,—kinematics. Besides these, the scientist has for his guidance, 'meta-physical' principles (such as the equality of action and reaction), which are guesses at law, more or less systematic, often childish,
