Phenomenology is the branch of philosophy that explores phenomena (observable, experiential events) and has principally been the concern of German philosophers and 20th-century French philosophers.
There are three distinct phenomenological schools: the dialectical, transcendental, and existential, all of which continue to have currency today and were prominent in the development of philosophy throughout the 20th century. Phenomenology is a descriptive approach to philosophy: It describes the world and the function of the mind, rather than prescribing the correct way to do a thing, as ethics does.
In his 1781 Critique of Pure Reason, perhaps the single most important text in Western philosophy, Immanuel Kant reacted to and rejected David Hume's empiricist claim that all ideas, all thoughts, were derived from "impressions," that is, from sensory experience.
Classical metaphysics, Kant argued, could not have been derived from sensory experience, and so he distinguished between phenomena, events as we experience them and objects as we observe them, and noumena, which exist independent of our perception of them and which we cannot therefore experience. A phenomenon is a representation of a noumenon; the noumenon for Kant is important primarily as a limiter, something against which to contrast the phenomenon.
Publishing several years after Kant's death, Georg Wilhelm Friedrich Hegel challenged Kant's noumenon/ phenomenon dichotomy, claiming in 1807's Phenomenology of Spirit that sufficient knowledge of phenomena can lead to complete apprehension of absolute truth. It was Hegel who coined the term phenomenology and who introduced the form of logic he called speculation and that is now referred to as Hegelian dialectics.
Most of the discussion in phenomenology, though, has been between the transcendental and existential schools. Transcendental phenomenology begins with Edmund Husserl, whose mentor Franz Brentano had taught that all perception is flawed and so, too, the conclusions drawn from it.
For Brentano and Husserl, absolute truths were unreachable because the mind was a flawed instrument; they recalled Hume in their description of consciousness as always "intentional." Intentionality in this respect includes the notion that every thought, every idea or feeling, is focused on some physical object.
In the 20th century, Martin Heidegger and the existential phenomenologists who followed him rejected Husserl's phenomenology. Heidegger was interested in the history of philosophy and the gaps he saw in its conversation about the world, particularly its failure to address what it means to be.
Altering the Husserl-Brentano model of intentionality, Heidegger said that consciousness is not simply "about" something, it is always caring about something. The experience of a thing is the feeling of that thing's relevance and importance.
By this time, phenomenology had become a concern to philosophers at large, not simply in the German schools. The French philosopher Henri Bergson wrote about perceptions of causality—a concern that had driven the works of Hume and Kant—and on the meaning of comedy and laughter; his influence on French philosophy combined with the growing interest in German phenomenology would shape much of the next century, from Jean-Paul Sartre to Michel Foucault to Jacques Derrida.