All of these resources are finite
All of these resources are finite. Knowledge, energy and purchasing power are replenishable. Time is the only resource that is finite that cannot be replenished, and therefore the most valuable.
— from Before Takeoff (front matter)
In the book
Everyone has such a purpose; it is usually hidden inside the things you are genuinely good at, and the surest way to use it is in the service of other people. Spend your hours with care, because of all your resources — knowledge, energy, money — only time cannot be replenished, which makes it the most valuable thing you own. Yesterday is history and tomorrow is a mystery; today is a gift, which is why it is called the present. — Before Takeoff (front matter)