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.
This dialectic, initiated by Euclides, became more and more opposed to the testimony of experience; in the hands of Eubulides and Alexinus it degenerated into hairsplitting, mainly in the form of the reductio ad absurdum.
The challenge must be countered in a reductio ad iinpossibile in which the dilemma is put.
Reid thus takes Hume's scepticism as, on its own showing, a reductio ad impossibile (see Hume, ad fin.) of accepted philosophical principles, and refuses, accordingly, to separate Hume from his intellectual progenitors.