What is interesting about the classic list under Rick_D's post above is that food is listed in the 10 essential list, but not water. You can get a lot farther with water and no food than the reverse. It IS listed under the 10 essential systems however.

I can't imagine going backpacking without a real knife. I can make a lot of useful stuff using that knife. We learned how to make all sorts of things in my first Boy Scout Troupe, including how to make bearpaw style snowshoes using willow branches. And I have had to give up a pocket knife flying before. Forgot it was in my pocket!