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Phenomenology is a current in philosophy that takes the intuitive experience of phenomena (what presents itself to us in conscious experience) as its starting point and tries to extract the essential features of experiences and the essence of what we experience. It stems from the School of Brentano and was mostly based on the work of the 20th century philosopher Edmund Husserl, and was developed further by philosophers such as Martin Heidegger, Maurice Merleau-Ponty, Max Scheler, Hannah Arendt, and Emmanuel Levinas. As such, phenomenological thought influenced the development of existential phenomenology and existentialism in France, as is clear from the work of Jean-Paul Sartre and Simone de Beauvoir, and Munich phenomenology (Johannes Daubert, Adolf Reinach in Germany and Alfred Schütz in Austria).
While the term "phenomenology" was used several times in the history of philosophy before Husserl, modern use ties it more explicitly to his particular method.
Later usage is mostly based on or (critically) related to Husserl's introduction and use of the term. This branch of philosophy differs from others in that it tends to be more "descriptive" than "prescriptive".
Husserl derived many important concepts that are central to phenomenology from the works and lectures of his teachers, the philosophers and psychologists Franz Brentano and Carl Stumpf. An important element of phenomenology that Husserl borrowed from Brentano was intentionality, the notion that the main characteristic of consciousness is that it is always intentional. Intentionality, which could be summarised as "aboutness", describes the relationship between mental acts and the external world. Every mental phenomenon or psychological act is directed at an object — the intentional object. Every belief, desire, etc. has an object to which it refers: the believed, the desired. The property of being intentional, of having an intentional object, is the key feature which distinguishes mental/psychical phenomena from physical phenomena (objects), because physical phenomena lack intentionality altogether. Intentionality is the key concept by means of which phenomenological philosophy attempts to overcome the subject/object dichotomy prevalent in modern philosophy.
In the Logical Investigations his first major work, still under the infuence of Brentano, Husserl still conceives of phenomenology as descriptive psychology. Husserl analyzes the intentional structures of mental acts and how they are directed at both real and ideal objects. The Logical Investigations begin with a devastating critique of psychologism i.e. the attempt to subsume the a priori validity of the laws of logic into psychology. Husserl establishes a separate field for research in logic, philosophy and phenomenology, independently from the empirical sciences.
Some years after the publication of the Logical Investigations, Husserl made some key elaborations which led him to the distinction between the act of consciousness (noesis) and the phenomena at which it is directed (the noemata).
What we observe is not the object as it is in itself, but how and inasmuch it is given in the intentional acts. Knowledge of essences would only be possible by "bracketing" all assumptions about the existence of an external world and the inessential (subjective) aspects of how the object is concretely given to us. This procedure Husserl called epoché.
Husserl in a later period concentrated more on the ideal, essential structures of consciousness. As he wanted to exclude any hypothesis on the existence of external objects, he introduced the method of phenomenological reduction to eliminate them. What was left over was the pure transcendental ego, as opposed to the concrete empirical ego. Now (transcendental) phenomenology is the study of the essential structures that are left in pure consciousness: this amounts in practice to the study of the noemata and the relations among them. German philosopher Theodor Adorno criticised Husserl's concept of phenomenological epistemology in his metacritique "Against Epistemology", which is anti-foundationalist in its stance.
After Husserl's publication of the Ideen in 1913, many phenomenologists took a critical stance towards his new theories. Especially the members of the Munich group distanced themselves from his new transcendental phenomenology and preferred the earlier realist phenomenology of the first edition of the Logical Investigations.
Existential phenomenology differs from transcendental phenomenology by its rejection of the transcendental ego. Merleau-Ponty objects to the ego's transcendence of the world, which for Husserl leaves the world spread out and completely transparent before the conscious. Heidegger thinks of conscious being as always and already in the world. Transcendence is maintained in existential phenomenology to the extent that the method of phenomenology must take a presuppositionless starting point - transcending claims about the world arising from, for example, natural or scientific attitudes or theories of the ontological nature of the world.
While Husserl thought philosophy to be a scientific discipline that had to be founded on a phenomenology understood as epistemology, Heidegger radically changed this view.
Heidegger himself phrases their differences this way:
According to Heidegger philosophy was not at all a scientific discipline, but more fundamental than science itself. Therefore, instead of taking phenomenology as prima philosophia or foundational discipline, he took it as a metaphysical ontology: "being is the proper and sole theme of philosophy". While for Husserl in the epochè being appeared only as a correlate of consciousness, for Heidegger being is the starting point. While for Husserl we would have to abstract from all concrete determinations of our empirical ego, to be able to turn to the field of pure consciousness, Heidegger claims that: "the possibilities and destinies of philosophy are bound up with man's existence, and thus with temporality and with historicality".
(NB: Heidegger's quotes taken from The Basic Problems of Phenomenology (1954), published by Indiana University Press, 1975. Introduction, p. 1 - 23 reproduced at www.marxists.org.)