Let's look at Franz Brentano's ideas, first. This German philosopher (January 16, 1838 to March 17, 1917) is famous for his theory involving, in his phase "intentional inexistence". By inexistence he means the mental wold. According to Franz Brentano, the essential characteristic of the mental is to be about something, to be directed toward something external. He thus means to avoid millenia of discussion about the relation of words and world.
His major work, later translated as Psychology from an Empirical Standpoint was published in 1874. For Brentano intention does not mean a human purpose, but a directionality to the thought. (The next quote is from a 20th century translation of a collection of Brentano's writing). Every thought, he said is about something--- outside-- the mental world. This has implications for errors in thinking:
If I say of a dog that he is a cat then it is indeed the case that the subject (dog) and the predicate (cat) have separate existence and that in taking the dog to be a cat, I am judging falsely. .....[I]f there were no cat at all -- neither united with nor separated from the dog, -- my judgment would still be false.
Another source highlights the significance of Franz Brentano in terms of the history of philosophy:
The basic approach of phenomenology was first developed by Franz Brentano, who was influenced both by scholastic versions of Aristotelian thought and by the radical empiricism of Hume. The central concern of philosophy, Brentano supposed, is to understand the nature and content of awareness in ways that illuminate the distinction between the mental and the non-mental.
....Brentano proposed that every mental act be understood to have a doubly significant representational function, designating both itself reflectively and a phenomenal object intentionally. Indeed, this distinction between acts and their objects precisely delineates the crucial distinction for Brentano, since"intentionality is the mark of the mental." One and the same phenomenal object can be intended by mental acts of different modalities—believing, imagining, etc. Thus, Brentano held that although each intentional act is itself subjective, its intention is an objective thing or fact in the world.
Stanford Encyclopedia of Philosohpy sketches a biographical note, and we excerpt:
Franz Brentano was born on January 16, 1838 in Marienberg am Rhein, Germany, a descendent of a strongly religious German-Italian family of intellectuals (his uncle Clemens Brentano and his aunt Bettina von Arnim were among the most important writers of German Romanticism and his brother Lujo Brentano became a leading expert in social economics). He studied mathematics, poetry, philosophy, and theology in Munich, Würzburg, and Berlin. Already at high school he became acquainted with Scholasticism; at university he studied Aristotle with Trendelenburg in Berlin, and read Comte as well as the British Empiricists (mainly John Stuart Mill), all of whom had a great influence on his work. Brentano received his Ph.D. in 1862, with his thesis On the Several Senses of Being in Aristotle..
After graduation Brentano prepared to take his vows; he was ordained a Catholic priest in 1864. Nevertheless he continued his academic career at the University of Würzburg,... [H]e eventually became full professor in 1873. During this period, however, Brentano struggled more and more with the official doctrine of the Catholic Church, especially with the dogma of papal infallibility, promulgated at the first Vatican Council in 1870. Shortly after his promotion at the University of Würzburg, Brentano withdrew from the priesthood and from his position as professor.
After his Habilitation, Brentano had started to work on a large scale work on the foundations of psychology, which he entitled Psychology from an Empirical Standpoint. The first volume was published in 1874, a second volume (The Classification of Mental Phenomena) followed in 1911, and fragments of the third volume (Sensory and Noetic Consciousness) were published posthumously by Oskar Kraus in 1928.
Shortly after the publication of the first volume, Brentano took a job as a full professor at the University of Vienna, where he continued a successful teaching career. During his tenure in Vienna, Brentano, who was very critical towards his own writing, no longer wrote books but turned instead to publishing various lectures. The topics range from aesthetics ....[English title ...The Genius...], Das Schlechte als Gegenstand dichterischer Darstellung [Evil as Object of Poetic Representation] ...and issues in historiography to ...[The Origin of the Knowledge of Right and Wrong,...] in which Brentano laid out his views on ethics. The latter was Brentano's first book to be translated into English in 1902.
When in 1880 Brentano and Ida von Lieben decided to wed, they had to confront the fact that the prevailing law in the Austro-Hungarian Empire denied matrimony to persons who had been ordained priests – even if they later had resigned from priesthood. They surmounted this obstacle by temporarily moving to and becoming citizens of Saxony, where they finally got married. This was possible only by temporarily giving up the Austrian citizenship and, in consequence, the job as full professor at the University. When Brentano came back to Vienna a few months later, the Austrian authorities did not reassign him his position. Brentano became Privatdozent, a status that allowed him to go on teaching – but did not entitle him to receive a salary or to supervise theses. For several years he tried in vain to get his position back. In 1895, after the death of his wife, he left Austria.... In 1896 he settled down in Florence where he got married to Emilie Ruprecht in 1897.
Brentano has often been described as an extraordinarily charismatic teacher. Throughout his life he influenced a great number of students, many of who became important philosophers and psychologists in their own rights, such as Edmund Husserl, Alexius Meinong, Christian von Ehrenfels, Anton Marty, Carl Stumpf, Kasimir Twardowski, as well as Sigmund Freud. Many of his students became professors all over the Austro-Hungarian Empire....