When I used to do a lot more outdoor activities and trip-leading out of a summer camp, there were some basic survival rules and techniques that are important to understand when heading out into the wilderness. I think it's helpful to think about these things and what we do (our are able to do) when these four basic needs are satisfied on an ongoing basis: air, shelter, water, and food.
The rule of threes: Human beings, in general, can survive for three minutes without air (oxygen), 3 hours without shelter (when having access to air but exposed to the elements), three days without water (when sheltered), and three weeks without food (with water, air and shelter). By the way, I am not basing of this off of any direct empirical evidence though I am something exists that from which this is justified. These are also probably conservative estimates (well maybe except the one about air).
From a biological (or physiological) perspective, these four elements are what human beings need in order to physically exist.
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