In the critical history of philosophy theory and history are intertwined in order to comprehend the multi-layered meanings of the philosophical ideas and their relations. History of ideas cannot mean denying any specificity of this kind of ideas or attributing them a vague meaning, as they are considered the meeting point of multiple interdisciplinary perspectives. On the contrary, the ideas of philosophy must be reconstructed both in their theoretical significance and historical genealogy by studying the ways in which philosophers are in dialogue with other philosophers or construct images of them or of some currents of philosophy. In the Husserl’s definition of eidos or essence converge, among others, its reflections on Lotze’s interpretation of the platonic doctrine of the ideas and on the Kant’s concept of the idea as regulative principle.