[FQ] And I may add it is not an easy matter to explain to those who have not considered such questions. It is a matter of the correlation of the testimony of the sense-organs. A boy stands before me. I go to him and touch him, and pass my hands downwards from head to foot. Then I stand a little way off and look at him. His image on my retina is inverted. But as I run my eye over him I direct my eye downwards to his feet and upwards to his head. I am not conscious that the stimuli are running _upwards_ along the retinal image. Thus my eye-muscles and my other muscular and tactile sensations seem to tell me that he is one way upwards. The image on my retina tells me, though I am not conscious of the fact, that he is the other way upwards. But he cannot be both! The testimony of one sense has to give way. One standard or the other has to be adopted. Practically that of touch and the muscular sensations is unconsciously selected, and sight-sensations are habitually interpreted in terms of this standard. So long as the two are sufficiently accurately correlated, the practical requirements of the case are met. And it is well known that it is not difficult, with a little practice, to establish a new correlation. This is indeed done every day by the microscopist, for whom the images are all reversed by his instrument. He very soon learns, however, that to move the object, as seen, to the left, he must push it to the right. A new correlation is rapidly and correctly established. [FR] I use this term because the word "percept" is used in different senses by different writers, e.g. by Mr. Mivart and Mr. Romanes. [FS] "Let the perception be considered to be made up of x + y; x being the ego, or self, and y the object. The mind has the power of supplying its own - x, and so we get (through the imagination of the mind and the object) x + y - x, or y pure and simple" (Mivart, "On Truth," p. 135). Mr. Mivart devotes a whole section of this work to the defence of ordinary common-sense realism. The above assertion seems to contain the essence of his teaching in the matter. [FT] If it be said that the object does exist independently of man, though not in the phenomenal guise under which we know it, I would reply--Not so; for it is to the existence _under this phenomenal guise_ that we apply the word "object." In philosophical language, the existence, stripped of its phenomenal aspect, is called the _Ding an sich_. Its essential character is its independence of man; and hence its unknowability. [FU] I avoid, for the present, the use of the terms "abstraction" and "abstract idea" because they are employed in different senses by different authors.