Hiking equipment
According to Tom Brown, the basic plan for survival is in the order of shelter (including clothing), water, fire, and food. Cody Lundin writes about the "Rule of 3s"; this relates to human survival without basics: three minutes without air, three hours without shelter, three days without water, or three weeks without food.
Fresh water
Water is critical to the survival of all living organisms. Some organisms can thrive on salt water, but the great majority of higher plants and most mammals need fresh water to live.