To many men it has seemed that the inference is not an easy one to justify.  One may say:  We could have no ideas of things, no sensations, if real things did not exist and make an impression upon our senses.  But to this it may be answered:  How is that statement to be proved?  Is it to be proved by observing that, when things are present and affect the senses, there come into being ideas which represent the things?  Evidently such a proof as this is out of the question, for, if it is true that we know external things only by inference and never immediately, then we can never prove by observation that ideas and things are thus connected.  And if it is not to be proved by observation, how shall it be proved?  Shall we just assume it dogmatically and pass on to something else?  Surely there is enough in the experience of the plain man to justify him in raising the question whether he can certainly know that there is an external world.

13.  THE PSYCHOLOGIST AND THE EXTERNAL WORLD.—­We have seen just above that the doubt regarding the existence of the world seems to have its root in the familiar distinction between ideas and things, appearances and the realities which they are supposed to represent.  The psychologist has much to say about ideas; and if sharpening and making clear this distinction has anything to do with stirring up doubts, it is natural to suppose that they should become more insistent when one has exchanged the ignorance of everyday life for the knowledge of the psychologist.

Now, when the psychologist asks how a given mind comes to have a knowledge of any external thing, he finds his answer in the messages which have been brought to the mind by means of the bodily senses.  He describes the sense-organs and the nervous connections between these and the brain, and tells us that when certain nervous impulses have traveled, let us say, from the eye or the ear to the brain, one has sensations of sight or sound.

He describes for us in detail how, out of such sensations and the memories of such sensations, we frame mental images of external things.  Between the mental image and the thing that it represents he distinguishes sharply, and he informs us that the mind knows no more about the external thing than is contained in such images.  That a thing is present can be known only by the fact that a message from the thing is sent along the nerves, and what the thing is must be determined from the character of the message.  Given the image in the absence of the thing,—­that is to say, an hallucination,—­the mind will naturally suppose that the thing is present.  This false supposition cannot be corrected by a direct inspection of the thing, for such a direct inspection of things is out of the question.  The only way in which the mind concerned can discover that the thing is absent is by referring to its other experiences.  This image is compared with other images and is discovered to be in some way abnormal.  We decide that it is a false representative and has no corresponding reality behind it.