By observing this distinction, we are in a position to mark off an illusion of introspection from a fallacy of introspection.  The former differs from the latter in the absence of anything like a conscious process of inference.  Thus, if we suppose that the derivation by Descartes of the fact of the existence of God from his possession of the idea to be erroneous, such a consciously performed act of reasoning would constitute a fallacy rather than an illusion of introspection.

We may, then, roughly define an illusion of introspection as an error involved in the apprehension of the contents of the mind at any moment.  If we mistake the quality or degree of a feeling or the structure of a complex mass of feeling, or if we confuse what is actually present to the mind with some inference based on this, we may be said to fall into an illusion of introspection.

But here the question will certainly be raised:  How can we conceive the mind erring as to the nature of its present contents; and what is to determine, if not my immediate act of introspection, what is present in my mind at any moment?  Indeed, to raise the possibility of error in introspection seems to do away with the certainty of presentative knowledge.

If, however, the reader will recall what was said in an earlier chapter about the possibility of error in recognizing the quality of a sense-impression, he will be prepared for a similar possibility here.  What we are accustomed to call a purely presentative cognition is, in truth, partly representative.  A feeling as pure feeling is not known; it is only known when it is distinguished, as to quality or degree, and so classed or brought under some representation of a kind or description of feeling, as acute, painful, and so on.  The accurate recognition of an impression of colour depends, as we have seen, on this process of classing being correctly performed.  Similarly, the recognition of internal feelings implies the presence of the appropriate or corresponding class-representation.  Accordingly, if it is possible for a wrong representation to get substituted for the right one, there seems to be an opening for error.

Any error that would thus arise can, of course, only be determined as such in relation to some other act of introspection of the same mind.  In matters of internal perception other minds cannot directly assist us in correcting error as they can in the case of external perception, though, as we shall see by-and-by, they may do so indirectly.  The standard of reality directly applicable to introspective cognition is plainly what the individual mind recognizes at its best moments, when the processes of attention and classifying are accurately performed, and the representation may be regarded with certainty as answering to the feeling.  In other words, in the sphere of internal, as in that of external experience, the criterion of reality is the average and perfect, as distinguished from the particular variable and imperfect act of cognition.