There are arcane theoretical reasons for thinking that you are not the same person when you wake up. There are also arcane conceptual scenarios that prevent you from being absolutely certain that you are the same person. These considerations are no match for your everyday certainty that it is you, even if they are able to cause short-term anxiety. Your everyday certainty will return, with a faint residue of wonder that we cannot absolutely prove the things we know. That residue is part of the point of doing philosophy. It helps save us from the dogmatic arrogance people are prone to. Your professor was partly joking and partly trying to push you toward this philosophical lesson.

Some philosophers would say that there are 10 trillion and 1 (at least). After all, you are a person and none of your cells is a person, so you are a distinct thing from each of them, and so an additional thing to all of them. Other philosophers would say that "living thing" is not a count noun because "thing" is not a count noun, and you need to be more specific about what you are counting. Is it persons? Then there is one. Is it cells? Then there are 10 trillion. I am dissatisfied with both these responses. It seems to me that "thing" is as much a count noun as is "item" in a sign for a grocery line that says "Six Items or Less." And it seems to me wrong to think that strictly speaking one should be barred from taking a six-pack through the "Six Items or Less" line because it is seven items: one six-pack plus six cans. Instead, it seems to me that the six-pack just is the six cans (neglecting packaging), not something in addition. The very same portion of reality can be accurately counted two different...