Edmund Husserl, a mathematician who had drifted into philosophy under Franz Brentano, published Logische Untersuchungen in 1900-01. The book attacked psychologism — the claim that logic is reducible to descriptions of how minds happen to operate — and proposed instead that consciousness has structures of its own that can be studied directly. The method, codified in Ideen (1913), came to be called phenomenology: the systematic first-person description of conscious experience, with metaphysical assumptions about objective reality bracketed (epoché). Husserl thought phenomenology could give philosophy what mathematics already had: a rigorous, foundational method. He was surprised when his most brilliant student took the method in a direction he found alien. Martin Heidegger's Being and Time (1927) turned phenomenology onto the question of being itself.
Husserl's method has three signature moves. Intentionality (from Brentano) names the structural feature that consciousness is always consciousness of something — to be conscious is to be directed at an object; even hallucinations have intentional structure. Epoché is the bracketing of the natural-attitude assumption that the world is independently real, so phenomena of consciousness can be described as they appear. Eidetic reduction extracts essences from particular experiences by imaginative variation. Heidegger deliberately departed from Husserl, wanting to recover the question of being he thought Western philosophy since Plato had forgotten. Dasein (being-there) is his term for the human kind of being, distinguished by the fact that its own being is an issue for it. Dasein is always already thrown into a world it did not choose; it exists as being-in-the-world, not as a disembodied subject. The world is encountered first practically through engaged dealings with things-as-equipment — the hammer is something to hammer with before it is a brown object with mass. Maurice Merleau-Ponty's Phenomenology of Perception (1945) extended the method by taking the body as foundational. Jean-Paul Sartre's Being and Nothingness (1943) applied phenomenology to existentialism (bad faith, the look of the Other, radical freedom); Simone de Beauvoir's Le Deuxième Sexe (1949) applied it to the lived experience of being a woman; Emmanuel Levinas turned the method toward ethics — the face of the Other as the primary ethical encounter, prior to any system of rules.
Phenomenology has had a second life through cognitive science. Francisco Varela, Evan Thompson, and Eleanor Rosch's The Embodied Mind (1991) brought Merleau-Ponty into conversation with cognitive science and Buddhist meditation, founding the 4E cognition programme that has reshaped how cognitive science thinks about brain, body, and world. Predictive processing (brief 304) has phenomenological undertones — perception as active inferential engagement rather than passive reception. Heidegger's tech critique, The Question Concerning Technology (1954), has become an unexpectedly active reference in AI ethics: his diagnosis of modern technology as Gestell (enframing), reducing the world to standing-reserve for use, reads as eerily prescient of platform capitalism. Hubert Dreyfus's What Computers Can't Do (1972) used Heidegger to argue symbolic AI would fail because it could not access the pre-theoretical bodily background that makes human intelligence possible.