Moreover, when we have found an error in something of which we were previously certain, we do not as a rule abandon entirely the belief which misled us, but we seek, if we can, to modify it so that it shall no longer be demonstrably false. This is what has happened with perception. When we think we see an external object, we may be deceived by a variety of causes. There may be a mirage or a reflection; in this case, the source of the error is in the external world, and a photographic plate would be equally deceived. There may be a stimulus to the eye of the sort that makes us “see stars”, or we may see little black dots owing to a disordered liver, in which case, the source of the error is in the body but not in the brain. We may have dreams in which we seem to see all sorts of things; in this case, the source of the error is in the brain. Having gradually discovered these possibilities of error, people have become somewhat wary as to the objective significance of their perceptions; but they have remained convinced that they really have the perceptions they thought they had, although the common-sense interpretation of them is sometimes at fault. Thus, while retaining the conviction that they are sure of something, they have gradually changed their view as to what it is that they are sure of. Nothing is known that tends to show error in the view that we really have the percepts we think we have, so long as we are prudent in interpreting them as signs of something external. That is the valid basis for Descartes’ view that “thought” is more certain than external objects. When “thought” is taken to mean the experiences which we usually regard as percepts of objects, there are sound reasons for accepting Descartes’ opinion to this extent. Let us now take up Dr. Watson’s view. We shall find, if I am not mistaken, that his position also is to a very large extent valid. That being so, we shall seek to find an intermediate opinion, accepting what seems valid and rejecting what seems doubtful in the contentions of both protagonists.