Knowledge.

When are you able to say that you really know something? To say that you 'know' connotes a complete meaning that has been found. Otherwise you would only be able to say that you 'know something about something' rather than that you 'know something.' The last step to 'knowing' is a finalizing, puzzle piecing together of evidences that you may have gathered. Until then you can only know parts and, though you might know many of them, you do not have a glimpse of the whole.

This is why movies are so frightening when the antagonist is some shapeshifter, changing forms to what suits its needs. Real people, though, are shapeshifters too. Maybe they should also be frightening, then, but in fact that is their allure. A few years spent with someone in a relationship of any sort will allow you to look back on who they were when you first came to know things about them and notice how differently those two personas compare. At least, that is a hope. A person ought to always be changing as a result of learning and experience rather than sit in any dusty stillness.

The best we know of a person is what was. We can know the whole of another's past persona because it is no longer changing, it hardens in time's coldness. But those past personas, however many, are only parts of what is now, they are only a handful of evidences, and so the only firm item of knowledge we may have of another person is that they are changing. They are not the whole. Not yet.