Now I see a line about an inch long on the paper before me. It is a certain distance from my eyes. I shall concern myself for the present only with the line immediately perceived, which means for me so much sensation. If I move this line (which remains the same in sense third), nearer to me or farther away, I do not perceive the identical thing that I did before. My quantity of sensation is increased or diminished. If I keep the line at the same distance and change none of the conditions, the quantity of sensation remains presumably the same. The question arises, Is this line as actually experienced at this moment infinitely divisible or not? I can certainly conceive of it as divisible to some extent, for I see part out of part, and I can think of these parts as separated. But if this line were divided, the division would soon result in parts which could be seen, but which could not be seen to consist of part out of part. Were these apparently non-extended parts (they would remain the same in sense third), approached to the eyes, they, too, would be seen to consist of part out of part, but then I should simply have substituted for the apparently non-extended a representative which was extended. This would not prove that what was before in consciousness was extended and could be divided. Consciousness certainly seems to testify that any particular line in consciousness is composed of a limited number of indivisible parts, and when one adds to this reflection the consideration that a point moving over a given line does not appear to have an endless task before it, but soon arrives at the final term, one is irresistibly impelled to the conclusion that the parts of the line are not infinite, but that the division results in the indivisible, the simple element of sensation, which, joined with other such elements, makes an extended object, but which taken alone is not extended at all. The whole difficulty lies in keeping to the line and the parts with which one started. It is so easy to pass from sameness in sense first to sameness in sense third or sense fifth; it is so natural to bring an object which is, as we say, imperfectly seen, closer to the eye and thus substitute for what was seen before a new experience connected with it in the order of nature, confident that any system of relations derived from the latter may safely be carried over to all possible experiences connected with the former; one does this so instinctively that a man may very readily suppose that he is still busied about the apparently non-extended element with which he started, when he is in reality dividing and sub-dividing its representative, which is evidently extended. But the question is not whether, when one has divided a line until the parts cannot be seen to consist of parts, one may substitute for these parts what evidently does consist of parts, and go on dividing that. The question is, whether an apparently non-extended