Item 64 - Letter from W. E. Johnson to Henry Sidgwick

Admits that he may have spoken too strongly [see 94/63] 'in implying that the method of Reductio ad absurdum would necessarily be formulated as If P then Q If P then not Q _ not P' Refers to two enclosed examples (included) 'in which this would seem the natural analysis.' Claims to be unable to 'find any good case (except from Geometry) of the method.' Would like to send to Sidgwick 'a good case of such an argument', but cannot find one. Letter accompanied by examples of two of Euclid's proposals in which the method of reductio ad absurdum is used, both involving geometry.