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. We see, then, that error in the process of introspection is at least conceivable. And now let us examine this process a little further, in order to find out what probabilities of error attach to it. To begin with, then, an act of introspection, to be complete, clearly involves the apprehension of an internal feeling or idea as something mental and marked off from the region of external experience. This distinct recognition of internal states of mind as such, in opposition to external impressions, is by no means easy, but presupposes a certain degree of intellectual culture, and a measure of the power of abstract attention. _Confusion of Internal and External Experience._