Background: Despite being often taken as the benchmark of quality for diagnostic and classificatory tools, 'validity' is admitted as a poorly worked out notion in psychiatric nosology. Objective: Here we aim at presenting a view that we believe to do better justice to the significance of the notion of validity, as well as at explaining away some misconceptions and inappropriate expectations regarding this attribute in the aforementioned context. Method: The notion of validity is addressed taking into account its role, the (...) framework according to which it should be assessed and the specific contents to which it refers within psychiatric nosology. Results and Conclusions: The notion of validity has an epistemological thrust and its foremost role is distinguishing correct reasoning and truth from what is irrational or false. From it follows not only that 'validity' always refers to elements of knowledge and rationality such as arguments, inferences and propositions, but also that the appropriate frameworks to assess 'validity' are logics and scientific methodology. When the validity of a psychiatric diagnostic category is at stake, the contents to which it refers are those relevantly related to the notion of 'diagnostic concept'. The consequences of our reading on the notion of 'validity' are discussed vis-à-vis the challenges faced by psychiatric nosology in order to have its diagnostic categories validated. (shrink)