I can swim. We also say that I know how to swim, but that is misleading because it makes the whole business sound more intellectualized than it is. How it actually works is this: put me in water and I will begin to do the crawl. I will propel myself from one side of the pool purposefully. My body knows what to do and I do not drown.
If you handcuff me before throwing me in the water, I would not be able to swim. But I would not have lost the capacity to swim. My body would still know how. It would just have been prevented by circumstance. You may be a sadist for applying handcuffs, but I am still a swimmer.
Linguists rely on the handcuffed swimmer analogy to illustrate the sometimes elusive difference between competence and performance. I have a capacity for speaking English, yet there are any number of ways I may fail to speak English: I may slur, stumble over a word, or try unsuccessfully to shout over a jackhammer. None of them detracts from my status as an English speaker. They are accidental impediments. Handcuffs.
To linguists, the capacity is the thing. It is what we call competence and is what we study. Performance effects are the accidental noise that must be stripped away, like a physicist strips away the effect of friction to get at the pure laws of motion. In language and other human activities there appears to be a strong correspondence between competence/performance and inside/outside. My capacity to swim or to speak is something internal to me. Impediments are external. They may matter in practice, but they are not me.
This can be a difficult concept to convey, despite the fact that it is something we all want to believe. I am my potential. What I actually do is just a pale refraction of the real thing, what I could do. The true me, my authentic self, is the homunculus inside my skull, directing my body through an imperfect world.
However, very few of us are ever thrown into a pool handcuffed. If you encounter a non-swimmer, it is probably because that person never learned to swim. Their body does not know what to do. No amount of academic study can grant you the internal capacity to swim, only time in the pool can do that. To be a swimmer you must have actually swum in water at some point in your life. Multiple times. Successfully. To count as an English speaker, you must communicate with other English speakers in English on a regular basis. Likewise, the mensch must also exhibit actual, felt kindness, and the the casanova must unmistakably seduce.
Properties that we would like to attribute to the internal homunculus–swimmer, Anglophone, mensch–have meaning only in our interaction with others. Our true selves don’t live inside our heads. They live in the two feet of air around our heads, where the world impinges.