1. In his work he attempted to establish psychology as an independent science, as “the science of mental phenomena”.
2. Founder of Act Psychology, or intentionalism, which concerns itself with the mind's acts or processes ( perception, judgment, loving, and hating) rather than with the contents and states of mind
3. Brentano put forward the idea of activity, objectivity and integrity of mind.
4. Intentionality. He is best known for having introduced the notion of intentionality to contemporary psychology and philosophy.
Every mental phenomenon, every psychological act has content, is directed at an object and intentionality or ‘directedness’ of the mental as is a fundamental aspect of thought and consciousness
5. The property of being intentional, was the key feature to distinguish psychical phenomena and physical phenomena.
6. Brentano distinguishes three kinds of mental phenomena: presentations (perception and imagining), judgments (acceptance or denial) and phenomena of love and hate (emotions, feelings, desires and acts of will).
7. Method. He stated that methods of psychology should be rigorous and exact as the methods of the natural sciences.
Introspection means for him to describe what one directly experiences in spontaneous inner perception, focused on the acts of mind, not its content.
8. He denies the idea of unconscious mental acts, however admited that we can have mental acts of various degrees of intensity.
9.Theory of perception. He maintained that we can be absolutely sure of our internal perception, but external sensory perception, can only yield hypotheses about the perceived world, but not truth.
10. He made important contributions to metaphysics and ontology, ethics, philosophical theology and offers many original ideas in all the main branches of philosophy: time-consciousness, objectivity of value judgments in ethics and esthetics, judgement–predication distinction, existential judgments.
He wrote his dissertation in 1862 at Tübingen under the title "On the Several Senses of Being in Aristotle". Subsequently he began to study theology and entered the seminary in Munich and then Würzburg. In 1866 he wrote and defended his habilitation thesis "The Psychology of Aristotle, in Particular His Doctrine of the Active Intellect".
Brentano became a Catholic priest on August 6,1864 but left the Church in 1873 (As a strong opponent of papal infallibility).
He was a teacher at the University of Würzburg (1866-73), professor of philosophy (1874 -1880) and Privatdozent (1881-1895) at the University of Vienna.
He exerted a strong influence on his students: Alexius Meinong, Christian von Ehrenfels, Rudolf Steiner, T.G. Masaryk, Sigmund Freud, Carl Stumpf, Kazimierz Twardowski, Edmund Husserl.
In order to marry, in 1880 he had to give up his professorship and Austrian citizenship and move to Leipzig. In 1880 he married his wife Ida von Lieben (m.1880-1895).
After his retirement in 1895, in 1896 he moved to Florence in Italy, where he got married to Emilie Ruprecht in 1897.
When Italy entered war against Germany and Austria at the First World War, he transferring to Zürich, where he died in 1917.