This is a representation of Husserl's idea of phenomenology as a foundational theory of science. The following essays elucidate the main features of the phenomenological method as worked out by Husserl in the course of the development of his philosophy - starting from merely "descriptive" and going on to "transcendental" and "constitutive" phenomenology - in order to get access to the foundations of knowledge in general and of scientific knowledge in particular. Further essays deal with the Husserlian foundations of natural science, and the relations between phenomenology and psychology, as well as those between phenomenology and history. This second revised and enlarged edition contains two further essays: one deals with Husserl's never abandoned idea of phenomenology as a rigorous science and his further claim to restore phenomenological philosophy as "First Philosophy"; and the second is on the problem of crisis of the Western culture - which Husserl was concerned with during several periods of his life - and demonstrates the actuality of his phenomenology even for philosophy of science in our times.