68. III. Circular or Retrogressive Demonstration. -- The reasoning faculty ascends from effect to cause that it may descend again from cause to effect and account for the latter by the former. It describes as it were a circle, returning, in a manner, to the point from which it started. This process is called circular demonstration.

The circular demonstration sets out from a phenomenon the existence of which is established, but its nature only confusedly perceived; it starts again from the nature of the phenomenon, but only after acquiring a more distinct idea of it, which better explains observed effects.

The circular demonstration must not he confounded with the vicious circle.