For a circle, when the rays emanate from any point, the secondary caustic is a limacon, and hence the primary caustic is the evolute of this curve.
Most of the canons, however, which constitute the ancient law, and notably those which appear in the Decretum of Gratian, emanate fr.,m local councils, or even from individual bishops; they have found a place in the common law because the collections of canons, of which they formed the most notable part, have been everywhere adopted.
When the refracting curve is a circle and the rays emanate from any point, the locus of the secondary caustic is a Cartesian oval, and the evolute of this curve is the required diacaustic. These curves appear to have been first discussed by Gergonne.
Starting, then, from this fundamental distinction between judgments of existence and judgments of non-existence, we may hope to steer our way between two extreme views which emanate from two important thinkers, each of whom has produced a flourishing school of psychological logic.