The Characteristic of Dukkha - Different kinds of rupa

Bhikkhu: I have a question on satipatthana. I have read that among the twenty-eight rupas, there are rupas that cannot be seen, rupas that cannot impinge, subtle rupas, rupas that are far, and so on. Could you explain about this?
Sujin: There is only one rupa among the twenty-eight rupas that can be seen, and that is visible object that appears through the eyesense. Visible object can be seen and it is among the rupas that impinge or contact (sappatigha rupas). There are other impinging rupas: the sense objects, apart from visible object, which are sound, odour, flavour and tangible object, consisting of solidity (appearing as hardness or softness), temperature (appearing as heat or cold) and motion (appearing as motion or pressure). Furthermore, there are the senses that can be contacted or impinged on, namely, eyesense, earsense, smellingsense, tastingsense and bodysense. These eleven rupas can impinge or can be impinged on, but they cannot be seen, whereas visible object can impinge and can be seen. The twelve impinging rupas are coarse rupas. They are also called rupas that are “near,”  because they can be investigated and known.
The sixteen other rupas among the twenty-eight rupas are the subtle rupas. They cannot be seen nor are they impinging. Subtle rupas are “far,” they cannot easily be discerned.
The Buddha explained about dhammas, realities, and people who develop satipatthana can verify them; they can know the characteristics of the dhammas that naturally appear, just as they are. However, the Dhamma is subtle and deep. For example, when a person learns that visible object is the reality appearing through the eyes, he may think that it is not difficult to understand this. But theoretical understanding is not the same as understanding of the characteristic of seeing when he sees. If he does not develop satipatthana so that panna becomes keener, he cannot realize the characteristics of nama and rupa as they are. When one sees, visible object is experienced through the eyes, but what one sees one takes for people, beings and different things. Then doubt arises and people wonder what visible object is like, what characteristic it has.
Visible object is the reality that appears when our eyes are open and there is seeing, not yet thinking about anything. Then the characteristic of visible object can appear naturally, as it is. As panna develops, one can become familiar with the fact that visible object which appears is not a being, person, self or anything else. Visible object is only the reality that appears through the eyes, that is its true nature. If people are not inclined to study and investigate the characteristic of visible object, it will be impossible for them to let go of the clinging to the idea they always had of seeing, namely, seeing people, beings or different things.