We have all had those days when we feel like we’ve given something all we’ve got, be it a mental or a physical challenge, and are running on empty; we just don’t think we can continue. These moments, however, are where the highest achievers are separated from the rest; they find a way to go on.

Wikipedia defines mental toughness as “a measure of individual resilience and confidence that may predict success in sport, education and the workplace.”

I would have expected the resilience component of the definition, but confidence adds another dimension. It suggests that our level of confidence in ourselves plays a large role in our ability to persevere. As such, mental toughness is also a key characteristic of having a growth mindset.

Richard Branson recently wrote about his own experience with mental toughness when he and his children participated in the Virgin Strive Challenge, a brutal one-month endeavor to raise money for charity and test conventional limits.

In his recap, Branson refers to an old Navy SEAL mindset that suggests that, even when we are at our most exhausted, we still have more in us to give. Referred to as the 40 percent rule, it asserts that when your mind is telling you you’re done, you’re really only about 40 percent done.

Like our cars, we all have a reserve tank that we can access. Whenever you’re faced with a seemingly insurmountable goal or obstacle and feel like you’re tapped out, remember the 40 percent rule. We’re all more resilient and mentally tough than we might believe and tapping this reserve can be the key to reaching our objective or goal.