All mental phenomena have in common, Brentano argues, “that they are only perceived in inner consciousness, while in the case of physical phenomena only external perception is possible” (Psychology, 91).
According to Brentano, the former of these two forms of perception provides an unmistakable evidence for what is true.
Since the German word for perception (Wahrnehmung), literally translated, means “taking-true”, Brentano says that it is the only kind of perception in a strict sense.
He points out that inner perception must not be mixed up with inner observation, i.e., it must not be conceived as a full-fledged act that accompanies another mental act towards which it is directed.
It is rather interwoven with the latter:
— in addition to being primarily directed towards an object, each act is incidentally directed towards itself as a secondary object. [NB]
As a consequence, Brentano denies the idea that there could be unconscious mental acts: SINCE every mental act is incidentally directed towards itself as a secondary object, we are automatically aware of every occurring mental act. [NB]
He admits, however, that we can have mental acts of various degrees of intensity. [βαθμών έντασης]
In addition, he holds that the degree of intensity with which the object is presented is equal to the degree of intensity in which the secondary object, i.e., the act itself, is presented. [ΝΒ] […]
Consequently, if we have a mental act of a very low intensity, our secondary consciousness of this act also will have a very low intensity.
From this Brentano concludes that sometimes we are inclined to say that we had an unconscious mental phenomenon when actually we only had a conscious mental phenomenon of very low intensity.
[Franz Brentano (Stanford Encyclopedia of Philosophy)](https://plato.stanford.edu/entries/brentano/#innerperception)