In one of his letters, Seneca describes himself as a “cold-water enthusiast.” He would “celebrate the new year by taking a plunge into the canal, who, just as naturally as I would set out to do some reading or writing, or to compose a speech, used to inaugurate the first of the year with a plunge into the Virgo aqueduct [present day Trevi Fountain].” But then he gives the real reason for his cold plunges: “The body should be treated more rigorously that it may not be disobedient to the mind.”

There’s a lot of interesting research about the health benefits of taking cold showers and going for a run and lifting weights. But the real reason to do these things is far more simple: it’s to make a statement about who is in charge.

Who is in charge? The courageous side or the cowardly side of you? The side that doesn’t flinch at discomfort or the side that desires to always be comfortable? The side that does the hard thing or the side that takes the easy way?

We challenge ourselves not to improve our immune system. Not to increase our metabolism. Not to reduce anxiety. Those things might be nice ancillary benefits but they are not the point. The purpose is to become the kind of person that can do it. How do you expect to do the big things that scare you—that scare others—if you haven’t practiced them? Why do you think you can endure the cold reception of a bold idea if you can’t even endure cold water? How can you trust that you’ll step forward when the stakes are high when you regularly don’t do that when the stakes are low? What gives you any confidence you’ll do the hard thing when people are watching if you can’t do that even when no one is watching?

The person who does something scary every day is less fearful than someone who can’t. The person who does something difficult every day is tougher than someone who doesn’t.