Q: I try to understand the logic behind the idea when an eye extends beyond the boundary line of face in a painting!

A: I think you may be using logic where it's not appropriate. It sounds like you're trying to comprehend cubism, which in it's essence is derived from African or tribal art of the early 20th century, and that progenitor of Modern art, 'ol Picasso, basically stole the stylistic form in order to create meaning beyond the normal bourgeois understanding, and, depending upon your opinion, transcend beyond the logical, or forever confuse people.
It would be worth it to check out other non-cubist forms of abstract art, like expressionism, abstract expressionism, minimalism, some forms of pop,...