Mark Cuban is an extremely successful guy. Not coincidentally, Mark Cuban is also an extremely smart guy. (And possibly to your surprise, Mark Cuban is also a very nice guy.)

Cuban has experience, he has connections, he has capital -- and he has a real dislike for one word:

"Relaxing is for the other guy," he writes in How to Win at the Sport of Business. "I may be sitting in front of the TV, but I'm not watching it unless I think there is something I can learn from it. I'm thinking about things I can use in my business, and the TV is just there."

That, in a nutshell, is the "secret" to Cuban's success. "It's not about money or connections," he says. "It's the willingness to outwork and outlearn everyone."

That's it. Success is based primarily on working hard -- incredibly hard.

Whenever I write about how success is often based on outworking other people -- both in terms of effort and in terms of hours spent -- many people respond with righteous indignation.

"What about work-life balance?" some ask me. "That's stupid: Work smarter, not harder," others say. "Get a clue. Hard work has nothing to do with it," I often hear.

Well, no. You can't have it both ways. You can work smarter, sure, but you also have to work harder.

Take successful entrepreneurs. Bill Gates evidently never slept, never changed clothes, never did anything but code and maneuver and strategize. In an industry filled with incredibly smart people -- where smart was and is commonplace -- he rose to the top by also working incredibly hard.

In fact, the common theme of almost every tale of entrepreneurial success is a person who worked countless 18- to 24-hour days. Replace the names and their stories sound almost identical. Even Tim Ferriss, lord of the four-hour workweek manor, stays incredibly busy with all his projects. (Of course, to Tim it doesn't feel like work.)

Or take successful people in other professions. Jeffrey Immelt, the CEO of GE, has worked 100-hour weeks for over 20 years. In a company filled with incredibly driven people -- where incredible drive is commonplace -- he rose to the top by also working incredibly hard.

Marissa Mayer used to sleep under her desk at Google. Tim Cook still wants to be first in, last out.

Or take sports. Forget practice and conditioning and everything else -- Tom Brady probably spends more time just watching film than the rest of us spend at work. In a sport filled with incredibly talented athletes, he'll one day be in the Hall of Fame because he also works incredibly hard.

Hard work has clearly paid off for all of those folks. Yet somehow people think hard work won't work for them. Maybe that's because of the whole "work smarter" thing?

Maybe not. Successful people already work smarter. They don't work mindlessly or inefficiently or ineffectively.

Where success is concerned working smarter is a given. Extremely successful people work smarter and they work harder.

Their effort is heroic, their payoff is often legendary, and we celebrate them for it.

"Wait," you say. "Luck plays a big part in success. So does timing. So do a lot of other factors."

This time you're right. But you can't control luck. You can't always control timing. You can't always control all those other factors.

But you can always control how hard you work.

Everyone defines success differently, as well they should. (Before you say your personal definition of success has everything to do with balance and personal relationships and nothing to do with mastering the business world, read this. I'm with you.)

But if you happen to define success by traditional measures like professional achievement and fortune and fame, hard work is the great equalizer.

You may not be smarter than everyone else. You may not be as talented. You may not have the same great connections, the same great environment, or the same great education.

If you're on the downside of advantage you may have none of those things.

But can always, always, always work harder than everyone else. Hard work can always be your difference.

Commit to outworking and outlearning everyone around you -- whether at work, or in your personal life, or wherever your definition of success leads you -- and you may not accomplish everything you hope for, but you will accomplish a lot more.

Like Cuban says, "Don't start a company unless it's an obsession and something you love. There are 24 hours in a day, and if people like their jobs, they will find ways to use as much of it as possible to do their jobs."