Thus the difference between introspective and other knowledge is only in our intention and in the degree of certainty. When we say, “I see a table”, we may intend to know an external object, but if so we may be mistaken; we are, however, actually knowing the occurrence of a visual percept. When we describe the same occurrence in the words “a certain coloured pattern is occurring”, we have changed our intention and are much more certain of being right. Thus all that differentiates our reaction when it gives introspective knowledge from our reaction when it gives knowledge of another kind is the elimination of a possible source of error. I come now to the question: Do we know our own thoughts in a more intimate way than we know anything else? This is a question to which it is difficult to give precision; it describes something that one _feels_ to be a problem without being able to say exactly what the problem is. However, some things can be said which may serve to clear up our feelings, if not our ideas. Suppose you are asked to repeat after a man whatever he says, as a test of your hearing. He says “how do you do?” and you repeat “how do you do?” This is your knowledge-reaction, and you hear yourself speaking. You can perceive that what you hear when you speak is closely similar to what you hear when the other man speaks. This makes you feel that your reaction reproduces accurately what you heard. Your knowledge-reaction, in this case, is the cause of an occurrence closely similar to the occurrence that you are knowing. Moreover, our inveterate naive realism makes us think that what we said was what we heard while we were speaking. This is, of course, an illusion, since an elaborate chain of physical and physiological causation intervenes between speaking and hearing oneself speak; nevertheless, the illusion re-enforces our conviction that our knowledge, in such a case, is very intimate. And it is, in fact, as intimate as it can hope to be, when our knowledge-reaction reproduces the very event we are knowing, or at least an event extremely similar to it. This _may_ be the case on other occasions, but we can only know, with any certainty, that it is the case when what is known is a percept. This accounts for the fact that our most indubitable and complete knowledge is concerning percepts, not concerning other mental events or events in the external world. Our reaction to a sound can be to make a similar sound, and if we are clever enough we can paint something very like what we see. But we cannot show our knowledge of a pleasure by creating for ourselves another very similar pleasure, nor of a desire by creating a similar desire. Thus percepts are known with more accuracy and certainty than anything else either in the outer world or in our own minds.