In the study we are constructing one branch of genealogical root in the development of detective genre - narration on criminal cases (Vidocq, De Quincey, Newgate Calendar) and famous trials (Pitaval). Pitaval is not only a collection of 'causes célébre' like it is in old literature about crime but the main topic is investigation and conviction of a criminal and finding him guilty by means of proves - at this stage legal knowledge takes effect. By means of Pitaval the connection of criminal topic and rational reflection applied by investigation of crime and proving guilty begins in literary discourse. In Pitaval's work particularly Enlightenment type of rationality connected with the conception of naturalness is concerned. The means of rational reflection allow come to conclusion about what natural acting is like. It is with respect to circumstances and this naturalness is a part of a rational argument. The naturalness and rational argument together produce truth. The voice of truth speaks first of all through the contradictions in the suspects' utterances (through their offends against the rules of reason) and not according to an eyewitness account. Here we read the initiative of the Enlightenment discredit of prejudice (Gadamer). This discourse transformation has been a necessary but not a sufficient condition of establishing the detective genre. Pitaval differentiates from a detective story by its linear composition although in Pitaval we can sometimes also come across the tendency to linear-reversible (detective) composition.