Now C is also a cube, for it is fourth from the unit. And C is to D as B is to C, therefore B has to C the ratio which a cube has to a cube. And C is a cube, therefore B is also a cube.
And since the unit is to A as A is to B, and the unit measures A according to the units in it, therefore A also measures B according to the units in itself. Therefore A multiplied by itself makes the cubic number B.