Brentano and Intentionality

Franz Brentano (1838 – 1917) defined psychology as “the science of mental phenomena”. He proposed a number of criteria to distinguish mental from physical phenomena including:
1. mental phenomena are exclusively the object of inner perception,
2. they always appear as a unity,
3. they are always intentionally directed towards an object.

By intentionality Brentano means that mental states are about, or directed towards, an object. You cannot just perceive; you have to perceive something. The same goes for intending, believing, imagining, recollecting, wishing, willing, desiring, loving, hating, judging, knowing, and so on.

Thus mental states refer beyond themselves to objects that may or may not exist; you can perceive an apple in front of you or you can imagine an apple that has no physical existence.