Consciousness, Brentano argues, always forms a unity. While we can perceive a number of physical phenomena at one and the same time, we can only perceive one mental phenomenon at a specific point in time. When we seem to have more than one mental act at a time, like when we hear a melody while tasting a sip of red wine and enjoying the beautiful view from the window, all these mental phenomena melt into one, they become moments or, to stick with Brentano’s terminology, divisives of a collective. If one of the divisives ends in the course of time, e.g., when I swallow the wine and close my eyes, but continue to listen to the music, the collective goes on to exist. Brentano’s views on the unity of consciousness entail that inner observation, as explained above, is strictly impossible, i.e., that we cannot have a second act which is directed towards another mental act which it accompanies. One can remember another mental act one had earlier, or expect future mental acts, but due to the unity of consciousness one cannot have two mental acts, one of which being directed towards the other, at the same time. As a consequence, unlike inner perception, these acts of inner observation are not infallible.
[Franz Brentano (Stanford Encyclopedia of Philosophy)](https://plato.stanford.edu/entries/brentano/#innerperception)
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