The article is concerned with the general characteristics of Aristotle's theory of a genus-differentia definition. The authors examine the validity of the definitions in the framework of legal language and present some objections against the definitions of per genus proximum et differentia specificam as they are considered by Aristotle. At the same time, through the objections to the position of genus-differentia definition critics, it is proved that in a number of cases Aristotle's theory is more preferable than the approach offered by Herbert Hart, the proponent of analytical legal philosophy. The argument of Peter Hacker is used to reinstate Aristotle's genus-differentia definitions.