Before he became arguably the greatest deep sea diver who ever lived, John Chatteron was a medic in Vietnam. What he saw there, as men died around him and as he was continually sent out into the jungle on essentially pointless missions in a probably unjustifiable war, were glimpses into deepest recesses of humanity.

What came from this were rules–certain heuristics for how to live, how to fight, how to escape death, how to face death, how to help others and how to be prepared for just about anything.

What he learned in Vietnam–not as a soldier, but as a medic whose job it was to save lives and not take them–guided Chatteron much later in life when he faced even more unimaginable stressors and trouble. He turned to them when he found himself in the water, diving just feet from the World Trade Center towers on 9/11, when he was one of the first to dive the wrecks of the Lusitania, the Britannic and the Titanic. You can imagine that he turned to them when became stuck inside a lost German U-Boat off the coast of New Jersey, and you can be damn sure he thought of it when he was later diagnosed with cancer (likely due to his exposure to Agent Orange).

You see, it’s often in times of deep shit that we discover truths about life. It’s when we realize that cliches and common sense and passed along wisdom are usually right and common for a reason. We can lean on them because they can support our weight.

John Chatterton’s rules deserve to be thought of in that league.

Here they are, adopted from the classic book, Shadow Divers.

— If an undertaking was easy, someone else already would have done it.

— If you follow in another’s footsteps, you miss the problems really worth solving.

— Excellence is born of preparation, dedication, focus, and tenacity; compromise on any of these and you become average.

— Every so often, life presents a great moment of decision, an intersection at which a man must decide to stop or go; a person lives with these decisions forever.

— Examine everything; not all is as it seems or as people tell you.

—It is easiest to live with a decision if it is based on an earnest sense of right and wrong.

— The guy who gets killed is often the guy who got nervous. The guy who doesn’t care anymore, who has said, “I’m already dead—the fact that I live or die is irrelevant and the only thing that matters is the accounting I give of myself,” is the most formidable force in the world.

—The worst possible decision is to give up.

These are literally battle hardened principles. They are rules for how to live not from some university professor but from someone who saw what life truly is–both good and bad, violent and peaceful, beautiful and terrifying.

They’ve been tested 200+ feet below sea level and in helicopters under fire in South Vietnam. I think it’s safe to say they may be of some value to you today, wherever you happen to be and whatever you happen to be doing.