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Mental toughness is many things and rather difficult to explain. Its qualities are sacrifice and self-denial. Also, most importantly, it is combined with a perfectly disciplined will that refuses to give in. It's a state of mind-you could call it character in action.

Vince Lombardi (2003). “What It Takes to Be #1 : Vince Lombardi on Leaders: Vince Lombardi on Leadership”, p.120, McGraw Hill Professional
Mental toughness is many things and rather difficult to explain. Its qualities are sacrifice and self-denial. Also, most importantly, it is combined with a perfectly disciplined will that refuses to give in. It's a