Edmund Husserl did not begin as a philosopher. He trained in mathematics under the great analyst Karl Weierstrass and wrote his earliest major work, the Philosophy of Arithmetic, trying to explain how mathematical concepts like number arise from ordinary mental acts of counting and grouping, an approach that drew a sharp rebuke from the logician Gottlob Frege for reducing objective mathematical truth to mere psychology.
That criticism pushed Husserl toward a more careful founding question, taken up across his Logical Investigations of 1900 and 1901: what is the actual structure of an act of consciousness that makes it about something, that gives it meaning, regardless of whether the something it is about actually exists. His answer centered on intentionality, the idea that consciousness is always consciousness of an object, and on a method he called the epoche, or bracketing, in which the philosopher deliberately sets aside, without denying, the ordinary assumption that objects simply exist independently out there, in order to examine the structures of experience exactly as they present themselves. Working this method out through decades at Gottingen and then Freiburg, he trained students who took it in directions he did not always endorse, most consequentially his chosen successor Martin Heidegger, whose Being and Time reoriented phenomenology toward questions of existence Husserl considered a departure from his own project.
In his final years in Freiburg, stripped of his university privileges under Nazi racial law because he had been born Jewish, he continued working in growing isolation until his death in 1938.