- Too little sleep
- Fasting, or skipping meals
- Shock and confusion after sustaining a serious injury
- Too much alcohol
- Too much coffee
- Legal, mood altering drugs administered by a doctor after surgery
- Legal, mood altering drugs administered by a doctor to treat depression
- Illegal, mood altering drugs administered by yourself
- The adrenaline rush during a confrontation
- Distress caused by physical pain or discomfort
- Delerium due to (oh, let's pick something..) vitamin B12 deficiency
- A stroke
- Alzheimer's disease
- Old age
Some have temporary effects, some permanent, but clearly they all are physical in nature and all affect your "I" (as DRH calls it), some profoundly so. Who am "I" but a particular walking, talking bag of meat? But that begs the question, which "I" would I be without this earthly body in heaven, hell, purgatory, sheol, hades, or jannah?
On the other hand, we are not simply crude masses of atoms. To be strictly reductionist about it, 99% of the human body is composed of just six elements. Yet, some blend of pure oxygen, carbon, nitrogen, phosphorous, hydrogen, and calcium doesn't make me or you. Nor are "you" a result of the specific cells that compose you. The average age of your body's cells is just 10 years. So unless you are rather young (and precocious, reading this -- hey, get your mom's credit card and buy my record), you are not even the "you" you were during the Clinton administration.
So, who are "you" and who am "I"? Science suggests that "we" are nothing more or less than uncommon, persistent patterns of common physical matter. If that isn't pure magic, I don't know what is.