Since the 1970's there has been a so called naturalist turn in the philosophy of science. This means that the quest for a universal method of knowledge of the sciences has been more or less supplanted in favor of a quest for analyzing historical and regional rationalities in the sciences. The turn has also meant an attempt to break away from the internalist focus of analysis, which has ignored the societal contexts and relations of scientific research. Ian Hacking has been one of the most prominent philosophers of science after the naturalist turn. His works, which owe much to Michel Foucault's philosophy, have been especially remarkable in the field of the human sciences. In my paper I will examine some of Hacking's ideas concerning the human sciences, namely historical ontology and styles of reasoning, in the context of emerging American educational psychology at the turn of the 20th century.When referring to his project of studying regional rationalities and forms of knowledge, Hacking speaks of "historical ontology". Ontology, of course, refers to the analysis of being. Historical ontology, in turn, implies that in the human sciences the categories and concepts for describing human behavior are historically contingent. The human sciences, he concludes, are always tied to societal and historical surfaces of emergence.However, Hacking notes that sciences are not just about describing phenomena. Research in the human sciences is also intervening; constructing and controlling human behavior through a melangé of social, intellectual and material techniques. In turn, the objects of human sciences - human behavior and the society - can affect research and the categories which are used to describe them. This process particular to the human sciences is what Hacking refers to as "looping".Hacking also asks how do the human sciences still claim their relative autonomy and stability over historical and societal contexts of emergence. In the field of human sciences there are various styles of reasoning, which consist of a coherent set of concepts, objects, instruments and methods. They also provide their own terms of validity and objectivity and are thus self-sustaining and self-validating rationalities.I will assess Hacking's ideas of historical ontology and styles of reasoning in relation to a particular case example, that of the emerging research in American educational psychology, which tried to establish itself as a natural science, yet providing useful knowledge for the domain of educational practises and school administration. I will also use the works of Michel Foucault, Bruno Latour and Thomas Gieryn to make up for some of the vague parts of Hacking's analytical apparatus. -Cremin, L.A. (1961). The Transformation of the School. New York: Alfred A. Knopf. -Danziger, K. (1990) Constructing the Subject. New York: Cambridge University Press. -Foucault, M. (2002) Archaeology of Knowledge. London: Routledge. -Gieryn, T. (1999) Cultural Boundaries of Science. Credibility on the Line. Chicago : University of Chicago Press. -Hacking, I. (2002) Historical Ontology. Cambridge: Harvard University Press. -Hacking, I. (2000) Social Construction of What? Cambridge : Harvard University Press. -Hacking, I. (1991) The Taming of Chance. Cambridge: Cambridge University Press. -Latour, B. (1986) Visualization and Cognition: Thinking with Eyes and Hands. Knowledge and Society: Studies in the Sociology of Culture Past and Present. 1-40 -Popkewitz, T. (1999) A Social Epistemology of Educational Research. In T. Popkewitz & L. Fendler (eds.) Critical Theories of Education. Changing Terrains of Knowledge and Politics. New York: Routledge. 17-42 -Popkewitz, T.S. (1991) A Political Sociology of Educational Reform. Power/Knowledge in Teaching, Teacher Education, and Research. New York: Teachers College Press. -Rose, N. (1996) Inventing Our Selves: Psychology, Power and Personhood. New York : Cambridge University Press .