any clear idea as to what kind of thing a thought is, and the object of this little book is to help us to conceive this.
There are some serious difficulties in our way, for our conception of space is limited to three dimensions, and when we attempt to make a drawing we practically limit ourselves to two. In reality the presentation even of ordinary three-dimensional objects is seriously defective, for scarcely a line or angle in our drawing is accurately shown. If a road crosses the picture, the part in the foreground must be represented as enormously wider than that in the background, although in reality the width is unchanged. If a house is to be drawn, the right angles at its corners must be shown as acute or obtuse as the case may be, but hardly ever as they actually are. In fact, we draw everything not as it is but as it appears, and the effort of the artist is by a skilful arrangement of lines upon a flat surface to convey to the eye an impression which shall recall that made by a three-dimensional