knowledge; in other words, is not adopted as the result of a process of
reasoning. What I believe with reference to my past history, so far as I
can myself recall it, I believe instantaneously and immediately, without
the intervention of any premise or reason. Similarly, our notions of
ourselves are, for the most part, obtained apart from any process of
inference. The view which a man takes of his own character or claims on
society he is popularly supposed to receive intuitively by a mere act of
internal observation. Such beliefs may not, indeed, have all the
overpowering force which belongs to illusory perceptions, for the
intuition of something by the senses is commonly looked on as the most
immediate and irresistible kind of knowledge. Still, they must be said
to come very near illusions of sense in the degree of their self-evident
certainty.

Taking this view of illusion, we may provisionally define it as any
species of error which counterfeits the form of immediate, self-evident,
or intuitive knowledge, whether as sense-perception or otherwise.
Whenever a thing is believed on its own evidence and not as a conclusion
from something else, and the thing then believed is demonstrably wrong,
there is an illusion. The term would thus appear to cover all varieties
of error which are not recognized as fallacies or false inferences. If
for the present we roughly divide all our knowledge into the two regions
of primary or intuitive, and secondary or inferential knowledge, we see
that illusion is false or spurious knowledge of the first kind, fallacy
false or spurious knowledge of the second kind. At the same time, it is
to be remembered that this division is only a very rough one. As will
appear in the course of our investigation, the same error may be called
either a fallacy or an illusion, according as we are thinking of its
original mode of production or of the form which it finally assumes; and
a thorough-going psychological analysis of error may discover that these