Mastering a foreign language

What is the meaning of mastering a foreign language?
Does the average native speaker truly master his own language? Can we cut up a language into pieces like "beginner's level", "intermediate level" or "proficiency level"?

To master a foreign language is an endless task, a dauntless chimera, an idle fancy.The same with our own mother tongue.
In our case we aim at mastering English and Spanish. Swahili is an easy, simple language to learn and communicate with, but we are never going to use it.
We can say we master a language when we can talk about any subject with ease, without gettiung tired, without making an intellectual effort. Of course, we will be restricted by our cultural and educational horizon. I, for one, cannot discuss biology or physics in either language because my ignorance of both subjects is abysmal.
The proof of the language is in the speaking. The proof of the language is in the writing. The proof of the language is in the understanding.
Beware of those who go around saying they have mastered several languages. To what extent?
In the meantime, let us not stop mastering English and Spanish, which are our target languages, without letting up, without ever resting in our linguistic laurels.