The concept of normality through history: a didactic review of features related to philosophy, statistics and medicine.

Normality characterises in medicine any possible qualitative or quantitative situation whose absence implies an illness or a state of abnormality. The illness concept was first a philosophical one. But the use of mathematics in the study of biological events, which began with Galton (1822-1911) and with Pearson (1857-1936), changed the frame of reference. In the second part of the 19th century mathematics was used to study the distribution of some biological characteristics in the evolution of the species. Around 1900, statistics became the basis for the study of the diffusion of the illnesses. Half a century later statistics made possible the transition from the description of single cases to groups of cases. Even more important is the concept of "normality" in laboratory medicine. In this field the search for the "perfect norm" was, and possibly still is, under way. The widespread use of statistics in the laboratory has allowed the definition, in a certain sense, of a new normality. This is the reason why the term "reference value" has been introduced. However, even the introduction of this new term has merely shifted the problem, and not resolved it.