Didascalicon, Book II: the sciences, their parts and their discipline
Chapter V. On the fourfold of the soul
The principle of this progression and regression is taught by number itself. Say three times one: it makes 3; say three times three: 9; say three times nine: 27; say three times 27: it makes 81. Behold, in the fourth degree unity first meets you, and you will see the same happen if you carry the multiplication on to infinity, so that unity always stands out in the fourth degree. Most rightly, moreover, is the simple essence of the soul expressed by unity, which is itself also incorporeal. The ternary, too, on account of the indissoluble bond of the middle unity, is fittingly referred to the soul, just as the quaternary, because it has two means and is therefore dissoluble, properly pertains to the body. The first progression of the soul, therefore, is that by which from its simple essence, which is figured by the monad, it extends itself into a virtual ternary, where it now desires one thing by concupiscence, scorns another by anger, and discerns between the two by reason. And rightly is it said to flow from the monad into the triad, because every essence is naturally prior to its power. Again, that the same unity is found three times in the ternary when multiplied signifies that the soul stands not by parts but whole in each of its powers. For we cannot call reason alone, or anger alone, or concupiscence alone a third part of the soul, since reason is neither other nor less in substance than the soul, nor anger other or less than the soul, nor concupiscence other or less than the soul; but one and the same substance receives different names according to its different powers. Then from the virtual ternary, by a second progression, it descends to rule the music of the human body, which is composed of the novenary; for because there are nine openings in the human body, everything by which the same body is quickened and ruled flows in and out according to natural temperance. This too is an order, because the soul naturally has its powers before it is mingled with the body. Afterwards, in the third progression, poured out already beyond itself through the senses, it extends to these visible things, which by 27, a solid number, extended in three dimensions after the likeness of body, are figured to be dispensed through infinite actions. But in the fourth progression, released from the body, it returns to the purity of its simplicity; and therefore in the fourth multiplication, where three times 27 has grown into 81, the monad appears at the summit, that it may clearly appear that the soul after the term of this life, which is designated by 81, returns to the unity of its simplicity from which it had first departed when it descended to rule the human body. That the term of human life naturally stands at eighty the prophet declares: If in the strong [in powers] they be fourscore years, and what is more of them is labor and sorrow (Ps. 89). Some think this fourfold progression is the quaternary of the soul of which we spoke above, and that it is called the quaternary of the soul to distinguish it from the quaternary of the body.