extended thing which is grasped through it? So our philosopher learns to distrust the immediate object of knowledge; to regard it as in some sense unreal as compared with what it represents; and to deny to it those properties which it apparently possesses. It is not extended, but it stands for extension; it is not colored, but it stands for color; it is not real, but it stands for reality. It is natural, however, for one who has gone thus far to go farther. When he reflects again upon the fact that he sees the tree of a different color at different distances; when he remembers that colors vary with the quality of the light by which they are seen; when he and his neighbor dispute concerning the true color of the one tree which he sees dotted with red leaves, and which his opponent claims to see of a uniform color; then he may well begin to ask himself what is the true color of the "real" tree, or whether it is certain that it has any color at all? May not, then, the "real" tree have only some of the qualities that we ordinarily attribute to trees? Perhaps, the space qualities? Or, worse yet, since some of the qualities that the ordinary man attributes to trees may be regarded as existing only in a shadowy way in our ideas, why may not the same be true of all the other qualities? How do we know that "real" trees are extended? How do we know that "real" extension must be assumed as the cause of the delusive apparent extension of our ideas, if it is true that "real" color need not thus be assumed as a cause of our sensations of color? What if the "real" thing exists only as an indescribable and incomprehensible somewhat, which we must assume as a cause of the immediately known, but of which we can know nothing more? When one has once begun this slippery descent, it is not easy to say where he may find a peg to stay him in his course.