ing that to a considerable extent we have selected our own experience.
Our second law affirms that we see the world not as it is but as modified by the individual peculiarities of our minds. The illusions that result are Bacon's well-known "idols of the den," doubtless the most fruitful of the four sources of error pointed out by that clear-headed philosopher. For our starting-point we may turn again to physics and physiology. Vibrations of the luminiferous ether of varying rapidity are perceived by the eye as a harmony of colors. Vibrations of the air of varying rapidity are perceived by the ear as a harmony of tones. Unless, now, we are prepared to say that the colors red or green, or that the tones a or a', are like or in any way similar to the motion of the ether or air; unless, further, we are prepared to say that, corresponding to the subjective harmony of colors and tones which we feel, there is an objective harmony of motions in the ponderable stuff, then we must admit that we have here cases of the great primary illusion of a phenomenal world of ideas like a noumenal world of things-in-themselves. With this ancient problem of perception we are not now concerned, but it serves as an illustration of our mental law of apperception. As the eye and the ear, each according to its structure, make over the manifold motions of the external world into sensations of light and sound, so the mind makes over the materials of knowledge into this or that product according to its peculiar constitution. Observe, however, this difference between the two cases. While the eye and the ear vary little in structure in different individuals, the variations in mental structure are endless, being determined by our environment, education, and inherited peculiarities. Color-blindness is comparatively rare and limited to a few colors; psychical blindness, in a greater or less degree, is a defect no man is free from.
The simpler illustrations of this law need not detain us. We put any new phenomenon into that class of our previous notions which it most closely resembles. A child who sees a cow for the first time calls it a horse, if familiar with horses. The same plant may be apperceived by a girl as a flower, by a farmer as a weed, by an old woman as an herb. The story of the precocious boy is in point. He sat under a tree as three strangers passed by. The first said, "What a fine stick of timber!" "Good-morning, carpenter," said the boy. The second, "What excellent bark!" "Good-morning, tanner." The third, "What a beautiful treetop!" "Good-morning, artist." He had correctly interpreted their vocations from their manner of apperceiving the tree. Our habits of thought, once started, grow on any food. We go by chance to hear a lecturer of an opposite party or sect, and come away confirmed in our own views. This law of mental inertia
