"It may be as well to caution you here that you must not speak of a line or quantity, by itself, as being incommensurable; this would be absurd. The diagonal of a square is not itself incommensurable, since it has, of course, its third part, fourth part, hundredth part, &c. and is therefore measurable by each of those parts; but as none of them will also measure the side, the two, considered together, are incommensurable: there exists no measure common to both. In the same way in reference to the circle—the circumference itself is not incommensurable any more than the diameter; for each has its fourth part, sixth part, &c. but it is incommensurable with its diameter: no length whatever can measure both."