Arithmetic, Certainty, Doubt, and Memory

I have been doing some arithmetic, that is, learning a little about the Riemann zeta function. As the days go by, I remember long forgotten math. But have I remembered or just learned anew? I don't think there is a distinct border between recall and learning anew. I see a pattern or technique and often think it is something I once knew. I could be mistaken though.

1 + 1 = 2. I am certain I remember that. But doubt can always creep in. Maybe, knowing something so simple is innate. I believe child development researchers have shown it plausible. Or is it merely because I have two apples sitting beside my computer as I write, and I have unconsciously worked out the sum anew?