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This paper is concerned with the reappraisal of Husserl’s ontology and epistemology, sketched in book one of Ideen. The main issue is Husserl’s theory of essence and essential insight. I present the fundamental distinction between facts and essences, and, over and above it, Husserl’s defense of an a priori knowledge based on essential insight as well as his partition of the whole realm of a priori knowledge into a formal set of material, regional ontologies. I will show how the theory of essential insight presented in Ideen gives rise to several criticisms, namely those made by Neo-Kantians, like Rickert and Natorp. In the last part, I will show how the mathematical concept of an invariant under a group of variations was the leading case for Husserl’s mature notion of eidetic insight.