Among the most shocking revelations that comes to an artist at some point in their career is the understanding that there is no such thing as "mastery" of one's art.
From below, looking up the mountain, it sure seems that way. Up there, perched at the apex, are The Masters. If we start climbing now, we might someday heave ourselves up, panting and shaking, into the same eyrie where they sit. There, we fold our legs and take our rightful place alongside them.
Except that this vision is a crock. It's not that The Masters are sitting up on the mountainside somewhere; it's that they're only Up There anywhere because they started flapping their arms and never stopped. The only way to join them is to flap one's own arms, and keep flapping. And even then you don't "join" them; you just flap along in some same general part of the sky as them.
My sense of this stems from insights gleaned by watching people regarded as "masters" in two walks of life. One was artistic creation; the other was spiritual discipline. It was actually some clarification about the habits and behaviors of the latter that came first, and helped me make sense of the former.
Zen teachers — the responsible and reputable ones, at least — discourage the use of the term "master". The word teacher is a better word, although even that still falls short in some ways. We still tend to think of teacher/student relationships as absolute one-up/one-down things, instead of something where the teacher and the student both provide things. A teacher who doesn't know how to learn from the experience of teaching his students (or, the experience of other teachers teaching other students) isn't much of a teacher.
Artists are masters in the same way. The only thing they really achieve mastery of is themselves; the only thing they can be better than is what they themselves were yesterday, or last year. Their mastery and their self-improvement does not map to anyone else, and isn't really supposed to.
Milton Glaser once made a parallel point involving design styles:
If you look at a Persian rug, you cannot say that less is more because you realise that every part of that rug, every change of colour, every shift in form is absolutely essential for its aesthetic success. You cannot prove to me that a solid blue rug is in any way superior. That also goes for the work of Gaudi, Persian miniatures, art nouveau and everything else.
Emphasis mine. Someone who is regarded as a "master" of SF or fantasy, and someone who is regarded as a "master" of literary fiction, is best seen as someone who has best transcended whatever limitations they themselves previously exhibited. Saying Dante is more of a master than Philip K. Dick is like complaining an orange isn't a very good automobile.
You can say that certain authors are better fits for certain kinds of material or storytelling. This is uncontroversial and obvious. But the concept of mastery, as something the creator on their own strives for, is an internal and subjective criterion. It's not a ranking that can be imposed from the outside, because these measurements only work when taken from within. You can only measure it versus what you were once upon a time, and the only measure there that matters is whether or not you're doing anything about it. What kind of thing did you read this year that you didn't read last year? What sort of art style did you expose yourself to that made you question some assumptions about how you could regard and frame things?
... when you are doing something in a recurring way to diminish risk or doing it in the same way as you have done it before, it is clear why professionalism is not enough. After all, what is desirable in our field, is continuous transgression. Professionalism does not allow for that because transgression has to encompass the possibility of failure and if you are professional your instinct is not to fail, it is to repeat success. Professionalism as a lifetime aspiration is a limited goal.
Maybe there's a way to talk about this objectively, by looking at who was able to shed how many of their own limitations over the course of their career, etc. But that strikes me less as a way to gain an understanding of one's own self-improvement and more as a way to just come up with a best-of list.
This is a process, meaning there is no such thing as mastery. There is only movement forward, stagnation, or regression.