common than illusions from the sense of sight. We all know how a cloud or sheet or coal may be converted by the Imagination into an image of something entirely different and visible only to the imaginer, although he supposes that others "must see it" too. But these are, so to speak, private illusions: the great public and, at one time, universal illusion, was the conviction that the sun and the stars move and that the earth does not move. There is scarcely any illusion more natural than this. Our senses give no indication whatever of the earth's motion; but they do indicate that the sun and the stars are moving. So complicated a process of reasoning, and so much experience, are needed before a man can realize (as distinct from repeating on authority) the causes for believing in the earth's motion that it is by no means surprising that, even now, only a minority of the human race believe that they are dashing through space at the rate of some thousands of miles an hour; and, except during the last three hundred years, the illusion that the earth is at rest was universal. Another common illusion from sight is that which leads us to suppose that, when we see anything in the air, a straight line from our eye towards the image which we see would touch the object itself: whereas, in reality, the image is raised by refraction so that in misty weather we see an object considerably higher than it is, and I suppose (to speak with strict exactness) we never "see" an object precisely where it is.
I have mentioned a few of the "illusions from the senses"; and now you will probably ask me what purpose they serve, how they can be called "wholesome," and how they "tend to the ultimate attainment of truth."
They appear to me to be "wholesome" because they represent and spring from a wholesome belief that "Nature will not deceive us; Nature does not change her mind; Nature keeps her promises." Sent into the
