After elucidating the controversy that accompanied the birth of laboratory medicine, the Authors define and examine the concepts underlying clinical methodology. The mental operations of the clinician are analyzed, a distinction being made between the processes of: a) "categorization" of the patient's disease, and b) "explanation" for pathologic phenomena. The diagnostic procedure is usually based above all on categorization. In the first phase of this procedure the clinician searches for data that are of the greatest possible "informative value", and then goes works out a certain number of "syndromic complexes". In the second phase, on the basis of these "groups of signs", the physician must formulate a certain number of diagnostic hypotheses and evaluate the probability of the presence of a particular disease, in view of the presence of particular signs. Finally, two fundamental arguments of clinician are dealt with: "the confirmatory argument" and "the falsifying argument", and the value of these two inferences in conferring certainty or a certain grade of reliability on clinical judgement is analyzed and discussed.