The history of physics, since the time of Maxwell, shows a record of vain efforts to reconstruct a materialistic monism. These attempts have failed because they involved a supposititious æther and atom which could no longer satisfy the growing body of experimental facts about radiant energy. After Maxwell predicted the existence of electro-magnetic radiation, an æther or atom possessing the ordinary attributes of matter became an absurdity. But our childish, or at least youthful, reluctance to admit our limitations is so great that we proceeded to repeat the hypothetical method once more by substituting an energiastic monism. Unmindful of Pascal’s dictum that man cannot investigate the world of either the infinitely small or the infinitely large, we are replacing a material atom and æther by Planck’s hypothesis of discrete and disembodied _quanta_, or atoms, of energy. Already there have arisen four insuperable difficulties. The hypothesis affirms action without specifying something to act, unless the word _energy_ is given all the physical attributes of matter. In which case a _quantum_ of radiant energy is merely our old friend, a corpuscle of light, masquerading under a new name. Again, it substitutes the principle of discontinuity of action for continuity, and one is puzzled to know from what the _quantum_ originates and in what it ends. And again, it creates the dilemma that light is simultaneously corpuscular and vibrational, since without the latter quality none of the phenomena of interference is explicable. Lastly, it drifts into a pure philosophy of idealism. My discussion would not be complete without a reference to the theory of relativity. It is significant that the earliest serious critique of Newtonian physics was made by Bishop Berkeley. With the greatest ingenuity and skill, he pointed out that objects can become known to us only _by_ the mind and that since light, sound, and motion are interpreted by the mind as essentially different, they cannot be synthesised by any experimental and objective process. So far Berkeley’s critique is thoroughly valid. He proceeded, however, to make the _non-sequitur_ that objects exist only _in_ the mind. If we accept this postulate, the logic of idealism is irrefutable; few, however, will accept it. Idealism fails because it ignores brute fact, and we accept the answer of Dr. Johnson who made his objection by merely kicking a large stone in his path.