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Johannes Kepler – Harmonices mundi – Book V (Frisch Vol V)

There follows, then, the straight line which, by the movement of a point located in the centre [of the sphere] to a single point on the surface, represents the first beginnings of creation, emulating the eternal generation of the Son in that the centre flows out towards infinitely many points of the whole surface, which, under the rule of most perfect equality, is formed and described by infinitely many lines; and this straight line is, needless to say, the element of corporeal form.

When widened, it [the straight line] already adumbrates the corporeal form itself, creating the plane. But when intersected by a plane, the sphere displays in this section the circle, the genuine image of the created mind, placed in command of the body which it is appointed to rule; and this circle is to the sphere as the human mind is to the Mind Divine, that is to say, as the line is to the surface; but both, to be sure, are circular. To the plane in which it is contained the circle is related as is the curve to the straight line, these two being mutually incompatible and incommensurable, and the circle beautifully fits into the intersecting plane (of which it is the circumscribing limit) as well as into the intersected sphere by way of a reciprocal coincidence of both, just as the mind is both inherent in the body, informing it and connected with corporeal form, and sustained by God, an irradiation as it were, that flows into the body from the divine countenance; whence it [the mind] derives its nobler nature.

As this situation establishes the circle as the underlying principle of the harmonious proportions and the source of their determinants, so does it demand the highest possible degree of abstraction because the image of the Mind of God dwells neither in a circle of any given size nor in an imperfect one such as are material and perceptible circles; and, what is the chief thing, because the circle must be kept as free (abstracted) from all that which is material and perceptible as the formulae of the curved line, the symbol of the mind, are separated and, as it were, abstracted from the straight, the simulacrum of the bodies. Thus we are sufficiently prepared for our task of deriving the determinants of the harmonious proportions, subject to the mind alone, from abstract quantities.