The Citta needs to see the heart objectively, so that it can realize it isn't it, but instead is the knowing of it.
The only true subjectivity is in the knowing.
Knowing the knowing leads to further knowing. Nothing else has this characteristic, ie everything else changes. Thus the idea that the pure Citta is unchanging.
In the unenlightened meditator , the base of knowing is centred on the physical Heart.