Diagnosis is widely used in many different disciplines to identify the nature and cause of a certain phenomenon. We present tL, a new logical framework able to formalise diagnostic reasoning, i.e., an hybrid learning technique based both on deduction and experiments. In this paper we introduce tL, a Labeled Modal Logic, garnishing with temporal and statistical information and a basic propositional language. After proposing examples on how tL effectively works, we sketch the main ideas about the full deduction system à la Prawitz we are currently developing.