You can know a thing. But that doesn’t necessarily mean that you know that you know it. And even if you know that you know it, it is not necessary that you can say it.
Once I had read in a book “if you can’t say it, you don’t know it”. It was one of those books that improve your grip on English language. This statement had increased my interest in that book and made me take the reading of that book much more seriously. But now I feel that this is such a stupid statement.
A child knows how to suck milk from the mother’s breasts. But can the child say it? Does it mean that the child doesn’t know it?