In his geometry, Euclid never multiplies a magnitude by a magnitude; for example, the line of length b is never multiplied by itself to produce the square b 2 . This is particularly clear in (1.prop.46), where he constructs a square (already defined in [1.def.22]): not even there, and nor anywhere else in the Elements , is it stated, assumed, or proved that the area of the square is the square of a side. Thus, for example, Pythagoras’s theorem, which follows at once with its converse (1.props.47–48), states that two squares are equal to a third one, and the well-known proof works by shuffling around regions of various shapes according to principles of congruence and composition; nowhere are area formulae involved. To make an analogy (and no more) with arithmetic, this theorem deals with, say, 9 +16 = 25, but not with 3 2 + 4 2 = 5 2 . In other words, in Euclid’s geometry t he square on the side is not the square of the side, or the side squared ; it is a planar region which has this size.... By contrast, in Euclid’s arithmetic numbers can be multiplied..... Thus, the algebraic version of his arithmetic is free of this objection. Ivor Grattan-Guinness (1996), Numbers, Magnitudes, Ratios, and Proportions in Euclid’s Elements : How Did He Handle Them?, Historia Mathematica 23/4, 355-375.