Like their ancient toga-wearing counterparts, modern philosophers continue to disagree on the nature of freewill. Do we really have any control over the choices we make and the things we desire, and if so, to what degree?Theories of freewill vary, but the ancient words of Plato still line up with our modern perceptions of temptation and willpower. The revered Greek philosopher argued that the human experience is one of constant struggle between the intellect and the body, between rationality and desire. Along these lines, true freedom is only achievable when willpower unchains us from bodily, emotional, instinctual slavery.

You can find similar sentiments throughout world religions, most of which offer a particular and often difficult path to rise above our darker natures.

And science? Well, science mostly agrees with all of this. Willpower is all about overcoming your natural impulses to eat cupcakes, skip your morning workout, flirt with the waiter, hit the snooze alarm and check your e-mail during a funeral.

Your willpower, however, is limited. If life were a video game, you'd see a glowing "willpower" or "ego" meter at the top of the screen next to your "life" meter. Successfully resist one temptation, and the meter depletes a little. The next temptation depletes the "willpower" meter even more, until there's nothing left at all.

Our modern scientific understanding of willpower in large part stems from a 1996 research experiment involving chocolate and radishes. Psychologist Roy Baumeister led a study in which 67 test subjects were presented with tempting chocolate chip cookies and other chocolate-flavored treats before a persistence-testing puzzle. Here's the catch: The researchers asked some of the participants to abstain from sweets and snack on radishes instead.

Baumeister's results told a fascinating story. The test subjects who resisted the sweet stuff in favor of radishes performed poorly on the persistence test. They simply didn't have the willpower left to resist slacking off.

The resulting paper, "Ego Depletion: Is the Active Self a Limited Resource?" inspired more than a thousand additional studies discussing everything from the influence of positive messages to the ego-sapping power of daily decisions 

Studies also show that cognitive capacity also affects our ability to hold out against temptation. Cognitive capacity is essentially your working memory, which you employ when resisting a temptation ... or holding a string of numbers in your head. A 1999 study from the University of Iowa professor Baba Shiv found that people tasked with remembering a two-digit number held out better than people remembering a seven-digit number when tempted with chocolate cake.