In contemporary philosophy of psychiatry, one increasingly hears the claim that if mental disorders turn out to be dimensional, the very idea of diagnostic validity may no longer be very important. Dimensionality here refers to the idea that symptoms vary along continuous spectra rather than forming discrete, sharply bounded categories. According to this view, once we accept the presence of broad dimensional structures, traditional efforts to determine whether psychiatric diagnoses correspond to real syndromes become outdated. I believe this perspective is mistaken.

Dimensionality by itself tells us very little about what exists in nature. In fact, almost everything in biology varies dimensionally. Hypertension, diabetes, osteoporosis, and most autoimmune diseases all exist along continua. Yet they remain valid medical constructs because they describe genuine syndromes with characteristic features, courses, and biological correlates. Variation across a spectrum does not preclude the existence of meaningful categories. In much of medicine, continuous variation and categorical distinction coexist.

Psychiatry is no different. The fact that........

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