Il libro2 di Euclide, dai commentatori viene titolato (dato che Euclide non da' titolo ai libri)
pag379 def2 Book2
the Pythagoreans and later Greek mathematicians exhibited different kinds of numbers as forming different geometrical figures.
Thus, says Theon of Smyrna (p. 36, 6—11), "plane numbers, triangular, square and solid numbers, and the rest, are not so called independently but in virtue of their similarity to the areas which they measure;
The product of two numbers was thus represented geometrically by the rectangle contained by the straight lines representing the two numbers respectively.
It only needed the discovery of incommensurable or irrational straight lines in order to represent geometrically by a rectangle the product of any two quantities whatever, rational or irrational; and it was possible to advance from a geometrical arithmetic to a geometrical algebra, which indeed by Euclid's time (and probably long before) had reached such a stage of development that it could solve the same problems as our algebra so far as they do not involve the manipulation of expressions of a degree higher than the second.
pag381 Heath vol1