The term "self-mutilation" occurred in a study by L. E. Emerson in 1913 where he considered self-cutting a symbolic substitution for masturbation. The term reappeared in an article in 1935 and a book in 1938 when Karl Menninger refined his conceptual definitions of self-mutilation. His study on self-destructiveness differentiated between suicidal behaviours and self-mutilation. For Menninger, self-mutilation was a non-fatal expression of an attenuated death wish and thus coined the term "partial suicide". He began a classification system of six types: