The voice that says you're done is running a conservative estimate

David Goggins failed his first attempt at Navy SEAL training by roughly seventy pounds of body weight, having grown up abused, barely literate, and overweight. He went on to become the only person in U.S. military history to complete SEAL training, Army Ranger School, and Air Force Tactical Air Controller training, and later set an official record for pull-ups in 24 hours: 4,030.

The idea he built around all of this, in Can't Hurt Me, is what he calls the 40 Percent Rule: when your mind insists you've hit your absolute limit, you've typically used about 40 percent of what your body can actually do. The other 60 percent doesn't disappear — it's withheld by a mental "governor" that exists to keep you safe, not to tell you the truth. There's real exercise science behind the mechanism: researchers studying perceived effort have shown the brain caps output well before the body's physiological ceiling.

Goggins's own application of this was brutal — he calls it "callusing" the mind through repeated, voluntary discomfort — and he's been candid that the extremes cost him physically. The number matters more than the extremity.

You don't have to run 200 miles to test it. You just have to notice, once, that the stop sign in your head arrived before your body actually needed to stop.