The object of this paper is to analyse the original conceptions of F. A. Hayek on the nature of facts of the social sciences. After having presented the main thesis of his youth's works on psychophysiology, we emphasize their links between those about economic epistemology and economic ontology : we thus insist on the place of subjectivism in Hayek's thought. An inconsistency is then noted and explained with the help of a precise definition of social ideology : our argument is that Hayek's subjectivism is incompatible with its liberalism.L'objet de cet article est d'analyser les conceptions originales de F. A. Hayek sur la nature des faits en sciences sociales. Après avoir présenté les principales thèses de ses travaux de jeunesse en psycho...