To find the line between you and the rest of the universe think about your senses. When you see, you look at things around you. It seems like what you see is not you. What you touch is also not you. You can feel it, then stop feeling it and so it is not you. If what we are feeling is your own knee or cheek, then, of course, it is you. What you hear is only sound. Birds, bells, the television and music are separate from you, but you hear them. Smell, again, when you smell bread baking in the oven, you do not confuse it with yourself. Last, but not least, is taste. Taste is where it gets interesting. When you taste something, that thing is in you. It is becoming a part of you. The sensation you get is coming from in your mouth, what you taste feels a lot like a part of you.
When you think about how these senses register in your brain, all of these senses are occurring in your head. As light or sound enters your head through your eyes and ears, your brain processes those waves and puts them into context. With taste and smell, it is chemicals. With touch, it is your own nerve impulses. There is an awareness somewhere that sees all this context and figures out what is you and what is not you. So there you are, a wild mix of light, sound, chemical, electrical, material and awareness all happening all at once all the time. Trying to draw a line around what of that is you and what is not can be tricky indeed. Anything that appears to you or occurs to you is inside you, unless your awareness is not in you. Who knows where that sits?