It is plain, indeed, that this is the position we have been, taking up throughout our discussion of illusion.  We have assumed that what is common and normal is true, or answers to what is objectively real.  Thus, in dealing with errors of perception, we took for granted that the common percept—­meaning by this what is permanent in the individual and the general experience—­is at the same time the true percept.  So in discussing the illusions of memory we estimated objective time by the judgment of the average man, free from individual bias, and apart from special circumstances favourable to error.  Similarly, in the case of belief, true belief was held to be that which men in general, or in the long run, or on the average, hold true, as distinguished from what the individual under variable and accidental influences holds true.  And even in the case of introspection we found that true cognition resolved itself into a consensus or agreement as to certain psychical facts.

Criterion of Illusion.

Now, it behoves us here to examine this assumption, with the view of seeing how far it is perfectly sound.  For it may be that what is commonly held true does not in all cases strictly answer to the real, in which case our idea of illusion would have to be extended so as to include certain common beliefs.  This question was partly opened up at the close of the last chapter.  It will be found that the full discussion of it carries us beyond the scientific point of view altogether.  For the present, however, let us see what can be said about it from that standpoint of positive science to which we have hitherto been keeping.

Now, if by common be meant what has been shared by all minds or the majority of minds up to a particular time, a moment’s inspection of the process of correcting illusion will show that science assumes the possibility of a common illusion.  In the history of discovery, the first assault on an error was the setting up of the individual against the society.  The men who first dared to say that the sun did not move round the earth found to their cost what it was to fly in the face of a common, though illusory, perception of the senses.[151]

If, however, by common be understood what is permanently and unshakably held true by men in proportion as their minds become enlightened, then science certainly does assume the truth of common perception and belief.  Thus, the progress of the physical sciences may be described as a movement towards a new, higher, and more stable consensus of ideas and beliefs.  In point of fact, the truths accepted by men of science already form a body of common belief for those who are supposed by all to have the means of testing the value of their convictions.  And the same applies to the successive improvements in the conceptions of the moral sciences, for example, history and psychology.  Indeed, the very meaning of science appears to be a body of common cognition to which all minds converge in proportion to their capabilities and opportunities of studying the particular subject-matter concerned.