It should be : "by which" not "in which". To give an example, if you look at a painting, the artwork is the painting. The artwork is not your impression or opinion of it.

It is a philosphical statement. It says that thing (the artwork) you are looking at (the mental act of apprehending it) is a real thing (the object of) and not something you are imagining ( the mental act in which it is apprehended). Even to native English speakers this is complete gobbledygook unless you have a basic understanding of philosophy.