be because there is more difference in the quality of the contacts in one case than in the other.  If they were on the same finger it might be said that they were stimulating the same general area, but since one is on one hand and one on the other this is impossible.  The subject does not think the two points are in the same place, because he feels two qualities and hence he infers two things, and he knows two things cannot be in the same place at the same time.  If the two contacts were of the same quality probably they would be perceived as one on account of the absence of difference, for the absence of difference is precisely the quality of oneness.

These facts, together with those mentioned before, seem to me to indicate that errors of localization are largely responsible for judgments which seem to be due to fusion or diffusion of sensations.  But they are responsible only in this way, they prevent the correction of the first impression.  I do not mean that a person never changes his judgment after having once made it, but a change of judgment is not necessarily a correction.  Often it is just the contrary.  But where a wrong judgment is made and cannot be corrected inability to localize is a prominent factor.  This, however, is only a secondary factor in the perception of number.  The cardinal point seems to me the following: 

Any touch sensation, no matter by how many objects it is produced, is one, and number is an inference based on a temporal series of sensations.  It may be that we can learn by association to infer number immediately from the quality of a sensation, but that means only that we recognize the sensation as one we have had before and have found it convenient to separate into parts and regard one part after the other, and we remember into how many parts we separated it.  This separating into parts is a time process.  What we shall regard as one is a mere matter of convenience.  Continuity sometimes affords a convenient basis for unity and sometimes it does not.  There is no standard of oneness in the objective world.  We separate things as far as convenience or time permits and then stop and call that one which our own attitude has determined shall be one.

That we do associate a sensation with whatever idea we have previously connected it with, even though that idea be that of the number of objects producing it, is clearly shown by some experiments which I performed in the laboratory of Columbia University.  I took three little round pieces of wood and set them in the form of a triangle.  I asked the subject to pass his right hand through a screen and told him I wanted to train him to perceive one, two, three and four contacts at a time on the back of his hand, and that I would tell him always how many I gave him until he learned to do it.  When it came to three I gave him two points near the knuckles and one toward the wrist and told him that was three.  Then I turned the instrument around and gave him one point