The aim of this article is to sketch a certain method of indirect reconstruction of the process by which mathematics as a deductive discipline emerged in ancient Greece. We try out this method in a reconstruction of Thales' mathematics, but the main aim for which this method has been developed is the work of Pythagoras. We consider the process of the emergence of mathematics as a process of the constitution of a new language in the framework of which one can carry out deductive proofs. Therefore we base the method of indirect reconstruction of the emergence of mathematics on the theoretical findings in the book L. Kvasz: Vedecká revolúcia ako lingvistická událosť (The Scientific Revolution as a linguistic event, Prague, Filosofia 2013)., Ladislav Kvasz., and Obsahuje poznámky a bibliografii