Spend enough time around something and it becomes familiar, and familiarity has a way of feeling like understanding. You know the surface so well that you assume you know the depths, and you stop looking. But familiar and understood are not the same thing, and the gap between them is exactly where refinement happens. Learning to treat the familiar as if it were unfamiliar is one of the more valuable orientations a person can develop.
We routinely overestimate what we understand
People believe they understand things far better than they actually do. Research on what is called the illusion of explanatory depth found that when people are asked to explain in detail how everyday things work, things they were confident they understood, they discover their understanding is far shallower than they assumed (Rozenblit & Keil, 2002). Familiarity produces the feeling of understanding without the substance of it. The confidence is real; the comprehension often is not. This is why something can feel completely known and still have most of its depth unexplored.
Experts see structure; familiarity sees surface
There is a real difference between knowing something on the surface and understanding its deeper structure. Classic research comparing experts and novices found that experts organize their knowledge around underlying principles while novices organize it around surface features, which is why experts can go deeper into the same material that beginners think they have already grasped (Chi, Feltovich & Glaser, 1981). Refinement is the movement from surface familiarity toward structural understanding, and it requires noticing that the familiar surface is not the whole thing.
The tell is whether you can still be surprised
A useful check on whether you actually understand something or are just familiar with it: can it still surprise you. If you return to a fundamental and find nothing new, you may genuinely understand it, or you may have stopped looking. The practitioner in refinement keeps finding more in the basics, which is itself evidence that the depth was there all along, hidden by familiarity. When the familiar stops surprising you, that is often a sign to look harder, not a sign you are done.
Treat the known as inexhaustible
The practical orientation of refinement is to approach the familiar with the working assumption that there is more in it than you have found. Not as blind optimism, but as a hypothesis the season keeps testing and keeps finding true. Go back to the thing you have done a thousand times and look at it as if for the first time. The depth tends to reward the return, which is exactly why mastery is a matter of going back rather than only moving on.
How to practice it
Periodically take a fundamental you consider settled and try to explain it in genuine detail, since that quickly reveals where your understanding actually thins out. Return to your earliest material with current eyes and notice what you can now see. Resist the pull to chase novelty when the depth in front of you is not exhausted. And treat the feeling of "I already know this" with mild suspicion, because that feeling is often familiarity rather than understanding.
Familiar is not understood. The things you know best may have the most left to teach you, and the willingness to treat the known as inexhaustible is what keeps the depth opening up. The world has no shortage of new things. Genuine understanding of the things you already have is rarer, and it is the work of this season.
References
Chi, M. T. H., Feltovich, P. J., & Glaser, R. (1981). Categorization and representation of physics problems by experts and novices. Cognitive Science, 5(2), 121–152.
Rozenblit, L., & Keil, F. (2002). The misunderstood limits of folk science: An illusion of explanatory depth. Cognitive Science, 26(5), 521–562.