We have now, perhaps, sufficiently reviewed sense-illusions, both of waking life and of sleep. And having roughly classified them according to their structure and origin, we are ready to go forwards and inquire whether the theory thus reached can be applied to other forms of illusory error. And here we are compelled to inquire at the outset if anything analogous to sense-illusion is to be found in that other great region of presentative cognition usually marked off from external perception as internal perception, self-reflection, or introspection. _Illusions of Introspection defined._ This inquiry naturally sets out with the question: What is meant by introspection? This cannot be better defined, perhaps, than by saying that it is the mind's immediate reflective cognition of its own states as such. In one sense, of course, everything we know may be called a mental state, actual or imagined. Thus, a sense-impression is known, exactly like any other feeling of the mind, as a mental phenomenon or mental modification. Yet we do not usually speak of introspectively recognizing a sensation. Our sense-impressions are marked off from all other feelings by having an objective character, that is to say, an immediate relation to the external world, so that in attending to one of them our minds pass away from themselves in what Professor Bain calls the attitude of objective regard. Introspection is confined to feelings which want this intimate connection with the external region, and includes sensation only so far as it is viewed apart from external objects and on its mental side as a feeling, a process which is next to impossible where the sensation has little emotional colour, as in the case of an ordinary sensation of sight or of articulate sound. This being so, errors of introspection, supposing such to be found, will in the main be sufficiently distinguished from those of perception. Even an hallucination of sense, whether setting out from a subjective sensation or not, always contains the semblance of a sense-impression, and so would not be correctly classed with errors of introspection. Just as introspection must be marked off from perception, so must it be distinguished from memory. It may be contended that, strictly speaking, all introspection is retrospection, since even in attending to a present feeling the mind is reflectively representing to itself the immediately preceding momentary experience of that feeling. Yet the adoption of this view does not hinder us from drawing a broad distinction between acts of introspection and acts of memory. Introspection must be regarded as confined to the knowledge of immediately antecedent mental states with reference to which, no error of memory can be supposed to arise.