The question of whether to view psychopathology as categorical or dimensional continues to provoke debate. We review the many facets of this argument. These include the pragmatics of measurement; the needs of clinical practice; our ability to distinguish categories from dimensions empirically; methods of analysis appropriate to each and how they relate; and the potential theoretical biases associated with each approach. We conclude that much of the debate is misconceived in that we do not observe pathology directly; rather, we observe its properties. The same pathology can have some properties that are most easily understood using a dimensional conceptualization while at the same time having other properties that are best understood categorically. We suggest replacing Meehl's analogy involving qualitatively distinct species with an alternative analogy with the “duality” of light, a phenomenon with both wave- and particle-like properties.