Eidetic reduction is a technique in Husserlian phenomenology, used to identify the essential components of the given phenomenon or experience. For each kind of phenomenon or experience, phenomenologists ask what the unique and essential components that make the phenomenon or the experience unique, distinguishable from all other kinds of phenomena or experiences. Our experience of time, for example, has unique characteristics. Eidetic reduction is used to identify those essential components that uniquely constitute our experience of time. Similarly, the method is used to identify all other kinds of experiences.
Eidetic reduction is used together with another key phenomenological method, phenomenological reduction. These techniques are parts of phenomenological researches for those who follow Husserlian phenomenology. Other phenomenologists do not necessarily accept Husserlian essentialist orientation and its methods to find “essences.” Heidegger, for example, whom Husserl expected to be his successor, challenged and rejected Husserl’s essentialist orientation and turned phenomenology into hermeneutic phenomenology. While Husserl, the originator of phenomenology, defined phenomenology as a presupposition free discipline, Heidegger argued that human understanding is always and already a form of interpretation and there is no interpretation free knowledge. Since eidetic reduction is a method to find essences, this method is also rejected if one refuses the existence of essences.
Eidos (εἶδος) is a Greek word meaning "image," "form," or "shape." The term became significant in Greek philosophy when Plato used it to refer to the ideal Forms or Ideas in his theory of Forms. Husserl adopted this term into phenomenology and used in a different context from Plato. Other than their essentialist orientation, there is little in common between Plato’s metaphysics and Husserl’s phenomenology.
Plato conceived Ideas as immutable and eternal beings that exist in a realm that transcends the world we live in. The phenomenal world is a “shadow” of eternal world of Ideas, and the task of philosophy was to become aware of and gain knowledge of the world of Ideas. Husserl does not posit this kind of Platonic world of essences transcending the phenomena. Essences are immanent in phenomena.
Eidetic reduction is a form of imaginative variation by which one attempts to reduce phenomenon into its necessary essences. This is done by theoretically changing different elements (while mentally observing whether or not the phenomenon changes) of a practical object to learn which characteristics are necessary for it to be it without being something else. If a characteristic is changed, and the object remains unchanged, the characteristic is unnecessary to the essence of the object, and vice versa.
Eidetic reduction requires that a phenomenologist examine the essence of a mental object, be it a simple mental act, or the unity of consciousness itself, with the intention of drawing out the absolutely necessary and invariable components that make the mental object what it is. This reduction is done with the intention of removing what is perceived and leaving only what is required.
This method can be illustrated by the example of Descartes's piece of wax (not as a mental object, but as a demonstration of the concept of reduction; see Descartes article). It appears to be opaque, flat, hard, extended to certain dimensions in space. It has a certain feel, smell, taste. Most of these qualities can be negated as necessary to the piece of wax continuing to be a piece of wax. If heated, it will continue to be the same piece of wax, the same molecules. However, the taste may change, the smell may become more noticeable, the texture will obviously change, it will become clear if heated to the point of melting etc. The only things that remain (its extension into space, chemical makeup, and mass) are the things that are required for the existence of that piece of wax.
One may, however, object to above identification of the essence of wax with its spatial extension and chemical components. Smell, taste, feeling, and texture can also be essential components of our experience of wax. “Free variation” and “eidetic reduction” in the above example may be guided by a preconception that spatial extension is the essence of a physical thing. In fact, Descartes’ discourses were guided by this preconception. Thus, other phenomenologists can legitimately find and present different essences of wax through their eidetic reduction.
Essences, thus, are not discoverable, definitive invariables. They are tentative claims phenomenlogicists make similar to a hypothesis that a scientist makes for their studies. What phenomenologists identify as essences at one point can turn out to be variables through further studies. Phenomenological determination is always tentative, and open to revision and modification. Eidetic reduction is not a method to find absolutely unchanging essences. In phenomenology, there is no claim for the ontological immutability of essences.
Husserl pursued genuine knowledge by attempting to be free from all kinds of presuppositions and preconceptions. He realized that even modern sciences were built upon a number of presuppositions. Husserl conceived phenomenology as a science that can disclose all kinds of presumptions built into our knowledge and can provide philosophical justifications for all sciences that pursue the objectivity of knowledge. Phenomenology for Husserl was a “presuppositionless” discipline, which he called “the science of all sciences.”
Some phenomenologists, starting with Heidegger, questioned the possibility of “presuppositionless knowledge.” Heidegger argued that human understanding is not possible without some sort of pre-knowledge, and understanding is necessarily interpretive. Drawing upon the tradition of hermeneutics of Dilthey and others, Heidegger re-interpreted phenomenology as a hermeneutic discipline. With Heidegger, phenomenology was divided into two streams between those who retained the Husserlian essentialist orientation and those who pursued hermeneutic phenomenology.
In addition, Heidegger questioned the concept of essence. He argued that concepts can be a product of cultural, historical traditions in the same way that languages that express these concepts are products of history. What one identifies as an essence can be merely an idea loaded with concepts of a particular intellectual tradition. Philosophical discourses in different traditions may not share the same conceptual distinctions. The difficulty of translating ideas among different linguistic communities, known as the problem of incommensurability, poses a challenge to the idea of the existence of universal essences. In addition, the human experience may be culturally conditioned without the individual being aware of it. Even the concept of color, for example, may not be the same between two culturally distinct communities. The idea of eidetic reduction is thus tied with other fundamental philosophical questions.