To attempt to explain the nature of matter, or heat, or light, the mechanism by which they act, or the method by which their energy is translated into sensation and thought, this, however, is to pass into the realm of metaphysics, or what Newton excellently called hypothesis. All such speculations are, at best, transitory, and, instead of predicting new lines of work, they lag behind the sure and steady advance of experimentation; they are constantly being revised to explain new phenomena after they are discovered. A convincing illustration of this criticism can be found in the history of the hypotheses of the nature of light. Physicists commonly assert that the corpuscular hypothesis retarded the advance of the subject of light for a century; their answer was to replace it by an equally metaphysical hypothesis of mechanical waves in an æther. After incessantly patching up this new conception during the next century, they are again returning to a corpuscular hypothesis, even more metaphysical and incomprehensible than its prototype. It seems impossible for us to learn that the trouble does not arise from the weakness of any particular variety, but lies in the nature of hypothesis, itself. We have created fictitious æthers, atoms, and electrons which bear no resemblance to sensible bodies; light is alternately a stream of corpuscles, or waves, or quanta of energy, or even a mathematical symbol; space is declared to be impenetrable except along certain curves; and time is confused with space. What one age proposes as a great advance is flung aside by the next which makes a new hypothesis whose only fate is to be rejected. So far have these speculations been carried that the dogma is seriously maintained that a false scientific hypothesis is valuable because in some mysterious way it leads to the discovery of truth. As we have advanced in sober experimental science, these hypotheses have become more and more abstruse and more and more dogmatic until the most recent of these dreamers, Whitehead, Eddington, Einstein, have pictured a phantasmagoria, instead of a world, as non-sensical as the hallucinations of the mediæval monk driven mad by the fevers of asceticism.