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The words 'psychological health' and 'psychological illness' play a considerable rôle in both scientific and pre-scientific medical literature. It is difficult to avoid the suspicion at times that a much too vague and many-sided use is made of them. No special apology is therefore needed if we propose to submit these concepts to a logical examination. Such a proceeding may not be very customary in medical literature, but we may hope through an explicit conceptual analysis to remedy a deficiency and to make of scientific language an instrument better adapted to the purposes of research. In any case, the concepts of 'psychological health' and 'psychological illness' seem to me to need and to deserve special investigation.
I believe that a considerable number of the concepts which we need for the description of bodily and psychological health and illness are to be found already minted in the science of genetics. Students of the mechanisms of heredity have found themselves obliged to pay careful attention to the variability of the forms and functions of the organism. As they encountered specially pronounced variations from the average in hereditary illnesses, whose mode of inheritance could often be traced more clearly than that of less conspicuous variations, they evolved a concept of 'illness' which seems to me of use outside the range of their particular purposes.
For our purposes we need to introduce the concepts of the 'reaction norm' ['Reaktionsnorm'] and the 'phenotype'.
The sum total of the characters which an organism displays at any given time composes its phenotype.3 We should call two animals 'phenotypically identical' if they were identical in all their characters. Of course, we never have occasion to do so. Animals and men alike only resemble one another in a limited number of characteristics, whether that number is great or