A single illusion of perception or of memory may suffice to give rise to a wholly illusory belief in a class of objects, for example, ghosts.

The superstitious beliefs of mankind abundantly illustrate this complexity of the sources of error.

And in the case of our every-day beliefs respecting real classes of objects, these sources contribute a considerable quota of error.

We may again see this by examining our ordinary beliefs respecting our fellow-men.
A moment's consideration will show that our prevailing views respecting any section of mankind, say our fellow-countrymen, or mankind at large, correspond at best to a very loose process of reasoning.

The accidents of our personal experience and opportunities of observation, the traditions which coloured our first ideas, the influence of our dominant feelings in selecting for attention and retention certain aspects of the complex object, and in idealizing this object,--these sources of passive and active illusion, must, to say the least, have had as much to do with our present solidified and seemingly "intuitive" knowledge as anything that can be called the exercise of individual judgment and reasoning power.
The force of this observation and the proof that such widely generalized beliefs are in part illusory, is seen in the fact that men of unlike experience and unlike temperament form such utterly dissimilar views of the same object.