Am I...?

Sometimes, when I least expect it, I get this strange thought.... and when I do I actually have to stop what im doing and ponder it over. It strikes me- I'm here. I'm alive. I'm actually on this earth along with millions of people like myself, going about life's activities. Or am I? Is this even real? Is this maybe the manifestation of someone's dream or imagination? Maybe any minute now that person will wake up and everything we know will vanish in a puff of logic... Which seems pretty much like me saying- What is real and what's not? Then I regain my senses and go about doing whatever it was I stopped, till the next time this notion comes floating back in my head to perturb me again for a while... this... who knows, maybe the reality......?