His 1874 Psychology from an Empirical Standpoint, considered his magnum opus, is credited with having reintroduced the medieval scholastic concept of intentionality into contemporary philosophy.

Originally a Catholic priest, Brentano withdrew from the priesthood in 1873 due to the dogmatic definition of papal infallibility in Pastor aeternus.

Working subsequently as a non-denominational professor, his teaching triggered research in a wide array of fields such as linguistics, logic, mathematics and experimental psychology through the young generation of philosophers who were gathered as the School of Brentano.

He wrote his dissertation in 1862 at Tübingen under the title Von der mannigfachen Bedeutung des Seienden nach Aristoteles (On the Several Senses of Being in Aristotle).

In 1866 he defended his habilitation thesis, Die Psychologie des Aristoteles, insbesondere seine Lehre vom Nous Poietikos (The Psychology of Aristotle, in Particular His Doctrine of the Active Intellect, published 1867), and began to lecture at the University of Würzburg.

He remained, however, deeply religious[15] and dealt with the topic of the existence of God in lectures given at the Universities of Würzburg and Vienna.

Among his students were Edmund Husserl, Sigmund Freud, Tomáš Masaryk, Rudolf Steiner, Alexius Meinong, Carl Stumpf, Anton Marty, Kazimierz Twardowski, and Christian von Ehrenfels and many others (see School of Brentano for more details).

While he began his career as a full ordinary professor, he was forced to give up both his Austrian citizenship and his professorship in 1880 in order to marry Ida Lieben (Austro-Hungarian law denied matrimony to persons who had been ordained priests even if they later had resigned from priesthood), but he was permitted to stay at the university only as a Privatdozent.

— Franz Brentano, Psychology from an Empirical Standpoint, edited by Linda L. McAlister (London: Routledge, 1995 [1874]), pp.

[6] (This concept is roughly equivalent to what is now called empirical psychology,[18] cognitive science,[18] or "heterophenomenology", an explicitly third-person, scientific approach to the study of consciousness.)

In fact he maintained that external, sensory perception could not tell us anything about the de facto existence of the perceived world, which could simply be illusion.

Hence he and many of his pupils (in particular Carl Stumpf and Edmund Husserl) thought that the natural sciences could only yield hypotheses and never universal, absolute truths as in pure logic or mathematics.

However, in a reprinting of his Psychologie vom Empirischen Standpunkte (Psychology from an Empirical Standpoint), he recanted this previous view.

The young Martin Heidegger was very much inspired by Brentano's early work On the Several Senses of Being in Aristotle.

[21] Brentano's work also influenced George Stout,[13] the teacher of G. E. Moore and Bertrand Russell at Cambridge University.