The debate on knowledge in New Education is generally dominated by two opposed Anglo-Saxon positions held by Dewey and Thorndike. This paper presents another line of division. Claparede and Vygotsky, two representative European figures of New Education are both scientists constructing a theory of psychological functioning, and heavily engaged in school reforms. Their conceptions of knowledge in education are nonetheless contrasted. We demonstrate it in analyzing their work from three points of view: the relationship between education and development; the nature of knowledge to teach; and the kind of knowledge necessary for teacher education. For Claparede, education follows natural development; knowledge to teach has to be useful and linked to everyday life; knowledge for the teacher is essentially knowledge on the child. For Vygotsky, education precedes development; knowledge to teach is systematic, different from everyday knowledge, transforming the relationship to its own psychic processes; knowledge for teachers is knowledge to teach and about teaching. Claparede's approach can be described as abstract negation of the traditional school; he wants a Copernican revolution, a completely different school linked to everyday life. Vygotsky's approach can be characterized as determined negation; he wants to build on the traditional school, maintaining and transforming knowledge organized systematically in formal disciplines. (Contains 101 footnotes.)