A good chemist masters chemistry. A good biologist masters biology. A good engineer masters engineering.
What about a good marketer, what does he master?
Being good at marketing is not just about knowing a lot about marketing. Mastering marketing principles is the first step (which is never finished, by the way), but what the best marketers do is master the science of motivation, behavior and social relations.
Throughout the marketing process, we spend a lot of time trying to understand the kind of people who are most likely to buy, but behavioural economics and decision research all suggest that 'where', 'what' and 'when' are at least as important as 'who'.
The best marketers don't just master marketing. They master the human condition.