Theorem, though there is very little to distinguish so simple a Theorem from an Axiom.
Min. Let us now consider the omissions, alterations, and additions, which have been proposed by your Modern Rivals.
§ 6. Omissions, alterations, and additions, suggested by Modern Rivals.
Euc. Which of my Theorems have my Modern Rivals proposed to omit?
Min. Without dwelling on such extreme cases as that of Mr. Pierce, who omits no less than 19 of the 35 Theorems in your First Book, I may say that the only two, as to which I have found anything like unanimity, are I. 7 and II. 8.
Euc. As to I. 7, I have several reasons to urge in favour of retaining it.
First, it is useful in proving I. 8, which, without it, is necessarily much lengthened, as it then has to include three cases: so that its omission effects little or no saving of space.
Secondly, the modern method of proving I. 8 independently leaves I. 7 still unproved.
Min. That reason has no weight unless you can prove I. 7 to be valuable for itself.
Euc. True, but I think I can prove it; for, thirdly, it shows that, of all plane Figures that can be made by hingeing rods together, the three-sided ones (and these only) are rigid (which is another way of stating the fact
