  Phenomenology as we know it was launched by Edmund Husserl in his Logical Investigations (1900–01). Two importantly different lines of theory came together in that monumental work: psychological theory, on the heels of Franz Brentano (and also William James, whose Principles of Psychology appeared in 1891 and greatly impressed Husserl); and logical or semantic theory, on the heels of Bernard Bolzano and Husserl’s contemporaries who founded modern logic, including Gottlob Frege. (Interestingly, both lines of research trace back to Aristotle, and both reached importantly new results in Husserl’s day.)