Anselm's motto, Credo ut intelligam, marks well the distance that has been traversed since Tertullian's Credo quia absurdum est.
4 He adduces it as a sort of reductio ad absurdum of Christians who would model life and cult on Christ and his apostles, unencumbered by later church traditions.
The Treatise is a reductio ad absurdum of the principles of Lockianism, inasmuch as these principles, when consistently applied, leave the structure of experience entirely " loosened " (to use Hume's own expression), or cemented together only by the irrational force of custom.
This was at once his strength and his weakness: his strength, for as a professional pleader he had learned how to deal with an adversary according to the rules of the art - to pull to pieces his theses, to reduce him ad absurdum, and to show the defects and contradictions of his statements, - and was specially qualified to expose the irregularities in the proceedings taken by the state against the Christians; but it was also his weakness, for it was responsible for his litigiousness, his often doubtful shifts and artifices, his sophisms and argumentationes ad hominem, his fallacies and surprises.
As a human person, I am body and soul; and the idealistic identification of the Ego with soul or mind, involving the corollary that my body belongs to the non-Ego and is no part of myself, is the reductio ad absurdum of idealism.
Credo, quia absurdum was applied, notably by the popular writers of the French Second Empire, in a fashion grotesquely literal enough to scandalize Tertullian himself.
Such a position is a reductio ad absurdum of the attempt to transcend the ultimate character of those intuitions and feelings which prompt men to benevolence.