The latter would be a simple presentative error, the former a compound representative error.
Finally, in order to complete this preliminary demarcation of our subject-matter, it is necessary to distinguish between an introspection (apparent or real) of a feeling or idea, and a process of inference based on this feeling.

The term introspective knowledge must, it is plain, be confined to what is or appears to be in the mind at the moment of inspection.
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.