Carlos Castaneda. The teachings of don Juan (№ 163)

...people closer to the end of the doctrine and unexpectedly will meet with the last of their enemy - old age. This is the most cruel of enemies, whose defeat is impossible, but you can drive away.
And now comes a time when a person is freed from the fear, overcame the clear, subdued force, but he was overcome by an unyielding desire to rest. If he succumbs to this desire to lie down and relax, if fatigue lull him, he will lose the last battle - the fourth enemy it will cast. The desire to relax will overcome all the clarity, all power, all knowledge.
But if a person will be able to overcome fatigue and pass your way to the end, then he will become a man of knowledge, at least for that brief moment when he will be able to ward off the last, invincible enemy. This moment of clarity, power and knowledge is enough.