My demonstration in the last resort is to his senses.  My thought makes me act on his senses much as he might himself act on them, were he pursuing the consequences of a perception of his own.  Practically then my thought terminates in his realities.  He willingly supposes it, therefore, to be of them, and inwardly to resemble what his own thought would be, were it of the same symbolic sort as mine.  And the pivot and fulcrum and support of his mental persuasion, is the sensible operation which my thought leads me, or may lead, to effect—­the bringing of Paley’s book, of Newton’s portrait, etc., before his very eyes.

In the last analysis, then, we believe that we all know and think about and talk about the same world, because we believe our percepts are possessed by us in common.  And we believe this because the percepts of each one of us seem to be changed in consequence of changes in the percepts of someone else.  What I am for you is in the first instance a percept of your own.  Unexpectedly, however, I open and show you a book, uttering certain sounds the while.  These acts are also your percepts, but they so resemble acts of yours with feelings prompting them, that you cannot doubt I have the feelings too, or that the book is one book felt in both our worlds.  That it is felt in the same way, that my feelings of it resemble yours, is something of which we never can be sure, but which we assume as the simplest hypothesis that meets the case.  As a matter of fact, we never are sure of it, and, as Erkenntnisstheoretiker, we can only say that of feelings that should not resemble each other, both could not know the same thing at the same time in the same way. [Footnote:  Though both might terminate in the same thing and be incomplete thoughts ‘about’ it.] If each holds to its own percept as the reality, it is bound to say of the other percept, that, though it may intend that reality, and prove this by working change upon it, yet, if it do not resemble it, it is all false and wrong. [Footnote:  The difference between Idealism and Realism is immaterial here.  What is said in the text is consistent with either theory.  A law by which my percept shall change yours directly is no more mysterious than a law by which it shall first change a physical reality, and then the reality change yours.  In either case you and I seem knit into a continuous world, and not to form a pair of solipsisms.]

If this be so of percepts, how much more so of higher modes of thought!  Even in the sphere of sensation individuals are probably different enough.  Comparative study of the simplest conceptual elements seems to show a wider divergence still.  And when it comes to general theories and emotional attitudes towards life, it is indeed time to say with Thackeray, ’My friend, two different universes walk about under your hat and under mine.’