Edmund Husserl, answered from the texts and cited to the page.
The question goes to the methodological heart of the whole enterprise, so let me lay it out with care. Every investigation of the psychic begins with the phenomenological reduction — the bracketing of the natural attitude's positing of the world, which yields access to the pure stream of lived experience as such. But what first becomes accessible in this way is the psychic as a factical, unrepeatable set of experiences belonging to one here-and-now self.1
A science of the psychic cannot rest content with mere facticity; it must reach what is invariant, necessary, and apodictic. The eidetic reduction is the further methodological step that accomplishes precisely this. The move is to prescind from all psychic facticity, treating the individual factical experience not as a datum to be catalogued but as an exemplar — a basis for free imaginative variation of possibilities.2
What survives every such variation, what cannot be thought away without the phenomenon itself dissolving, is the eidos, the essential form. A phenomenological analysis of the perception of a spatial thing, for instance, is in no way a report on perceptions that happen to occur or might be expected to occur empirically; it lays out the necessary structural system without which a synthesis of manifold perceptions as perception of one and the same thing could not be thought at all.3
The eidetic reduction thus converts the reductive-descriptive attitude into a genuinely scientific one. The phenomenological reduction reveals the phenomena of actual inner experience; the eidetic reduction discloses the essential forms that constrain psychical existence as such.4 In the language of Ideas I: the eidetic reduction leads from the factical psychological phenomenon to the pure essence, from empirical universality to the universality of an eidos — and the transcendentally purified phenomena it investigates are irreal, set outside any classification in the actual world, yet constituting the very conditions under which any world can be experienced.5
What this means concretely can be seen in the case of something as apparently simple as the color red. If I perform the phenomenological reduction and attend to pure immanence, I can actualize in pure seeing not this or that particular red, but red in specie — the identical universal that is seen in this or that instance. Once the species is given in this act of ideating seeing, it is senseless to doubt what the essence of red is: I see it, there it is.6
The eidetic reduction is simply the disciplined generalization of this capacity — extended from simple qualities to the full intentional structure of any kind of lived experience whatsoever. All phenomenological concepts and propositions require direct demonstration upon the phenomena themselves; the essential structures of the psychic are read off from the psychic directly, not inferred from causal hypotheses or statistical regularities.7
The upshot: phenomenological psychology, if it is to be a genuine science rather than a collection of empirical observations, can be realized only as reductive-eidetic — only as phenomenological — psychology.8 The same holds, at the higher transcendental level, for phenomenology itself. The entire investigation carried out for the sake of clarifying the principles that serve as the ideal conditions of the possibility of scientific objectivity moves within the sphere of essence, which constitutes itself on the underlying basis of singular phenomena within the phenomenological reduction.9
that which first of all becomes accessible in the performance of the phenomenological reduction is the pure psychic as a factical, unrepeatable set of experiences of one here-and-now self.Psychological and Transcendental Phenomenology and the Confrontation with Heidegger, pp. 155–156
the attitude of reduction to the pure psychic that initially shows up as an individual factical set of experiences must prescind from all psychic facticity. This facticity serves only exemplarily as a basis for the free variation of possibilities.Psychological and Transcendental Phenomenology and the Confrontation with Heidegger, pp. 155–156
the phenomenological analysis of the perception of spatial things is in no way a report on perceptions that occur factically or that are to be expected empirically. Rather, a phenomenological analysis means laying out the necessary structural system without which a synthesis of manifold perceptions, as perception of one and the same thing, could not be thought.Psychological and Transcendental Phenomenology and the Confrontation with Heidegger, pp. 155–156
The phenomenological reduction reveals the phenomena of actual internal experience; the eidetic reduction, the essential forms constraining psychical existence.Psychological and Transcendental Phenomenology and the Confrontation with Heidegger, p. 230
The corresponding reduction that leads from the psychological phenomenon to the pure 'essence' or that leads (in the sort of thinking involved in making a judgment) from the factual ('empirical') universality to the universality of an 'essence' is the eidetic reduction.Ideas I, p. 6
I actualize in pure seeing the sense of the thought red, red in specie, the identical universal that is seen in this or that; now the particularity as such is no longer meant, but rather red in general. If we in fact do this in a pure act of seeing, would it still make sense to doubt what red in general is, what is meant by 'red,' what it is according to its essence? We see it — there it is.The Idea of Phenomenology, pp. 45–46
All phenomenological concepts and propositions require direct demonstration upon the phenomena themselves... the essential structures of the psychic are read off from the psychic directly.Psychological and Transcendental Phenomenology and the Confrontation with Heidegger, pp. 155–156
The scientific exploration of the pure psychic, pure psychology, can be realized only as reductive-eidetic — that is, as phenomenological — psychology.Psychological and Transcendental Phenomenology and the Confrontation with Heidegger, pp. 155–156
The entire investigation carried out for the sake of clarifying these principles moves within the sphere of essence, which, in turn, constitutes itself on the underlying basis of singular phenomena within the phenomenological reduction.The Idea of Phenomenology, p. 46