In all, eight formulæ. The angles at the base of an isosceles triangle do not obviously come within any of these. The formulæ specify certain marks of equality and of inequality, but the angles cannot be perceived intuitively to have any of those marks. We can, however, examine whether they have properties which, in any other formulæ, are set down as marks of those marks. On examination it appears that they have; and we ultimately succeed in bringing them within this formula, “The differences of equal things are equal.” Whence comes the difficulty in recognising these angles as the differences of equal things? Because each of them is the difference not of one pair only, but of innumerable pairs of angles; and out of these we had to imagine and select two, which could either be intuitively perceived to be equals, or possessed some of the marks of equality set down in the various formulæ. By an exercise of ingenuity, which, on the part of the first inventor, deserves to be regarded as considerable, two pairs of angles were hit upon, which united these requisites. First, it could be perceived intuitively that their differences were the angles at the base; and, secondly; they possessed one of the marks of equality, namely, coincidence when applied to one another. This coincidence, however, was not perceived intuitively, but inferred, in conformity to another formula. For greater clearness, I subjoin an analysis of the demonstration. Euclid, it will be remembered, demonstrates his fifth proposition by means of the fourth. This it is not allowable for us to do, because we are undertaking to trace deductive truths not to prior deductions, but to their original inductive foundation. We must therefore use the premisses of the fourth proposition instead of its conclusion, and prove the fifth directly from first principles. To do so requires six formulas. (We presuppose an equilateral triangle, whose vertices are A, D, E, with point B on the side AD, and point C on the side AE, such that BC is parallel to DE. We must begin as in Euclid, by prolonging the equal sides AB, AC, to equal distances, and joining the extremities BE, DC.) FIRST FORMULA. _The sums of equals are equal._ A D and A E are sums of equals by the supposition. Having that mark of equality, they are concluded by this formula to be equal. SECOND FORMULA. _Equal straight lines being applied to one another coincide_. A C, A B, are within this formula by supposition; A D, A E, have been brought within it by the preceding step. Both these pairs of straight lines have the property of equality; which, according to the second formula, is a mark that, if applied to each other, they will coincide. Coinciding altogether means coinciding in every part, and of course at their extremities, D, E, and B, C. THIRD FORMULA. _Straight lines, having their extremities coincident, coincide_. B E and C D have been brought within this formula by the preceding induction; they will, therefore, coincide.