Plutarch, De animae procreatione in Timaeo
Goodwin, Ed.

The same may be said against the followers of Posidonius. For they seem not altogether to separate the soul from matter; but imagining the essence of limitations to be divisible in reference to bodies, and intermixing it with the intelligible essence, they defined the soul to be an idea [p. 351] (or essential form) of that which has extension in every direction, subsisting in an harmonical proportion of numbers. For (they say) all mathematical objects are disposed between the first intelligible and sensible beings; and since the soul contains the sempiternal nature of things intelligible and the pathetic nature of things subjected to sense, it seems but rational that it should consist of a substance between both. But they were ignorant that God, when the soul was already brought to perfection, afterwards making use of the limitations of bodies to form and shape the matter, confined and environed the dissipated and fleeting substance within the compass of certain surfaces composed of triangles adapted together. And it is even more absurd to make the soul an idea. For the soul is always in motion; the idea is incapable of motion; the one never to be mixed with that which is subjected to sense, the other wrought into the substance of the body. Moreover, God could be said only to imitate an idea, as his pattern; but he was the artificer of the soul, as of a work of perfection. Now enough has been already said to show that Plato does not assert number to be the substance of the soul, only that it is ordered and proportioned by number.