If there’s one scenario more than any other that prompts people to own a firearm, it’s probably home invasions. I don’t have hard data on that or anything, just an impression based on my experiences talking with people through the years.

I suspect my experiences and impressions aren’t unique, either.

In Missoula, Montana, a math professor found himself faced with a home invasion. Luckily, he was armed and could change the equation on the criminal.

A Missoula math professor thought his dog was chasing his cat around his home Monday morning when he heard some commotion from inside the house. When he started hearing louder-than-usual sirens and noticed a rapidly growing police presence around his property, however, his inclination shifted.

“I briefly opened my front door and learned authorities were seeking the whereabouts of one or more individuals,” Josef “Joe” Crepeau wrote in an open letter to the community. “My interpretation of the noise quickly changed. I felt I should convince myself the subject or subjects of the search were not [the cause of the noise] so I decided to search my home for intruders.”

Crepeau, 58, provided his letter to the Missoulian and elaborated in person Tuesday.

He said he grabbed his 9 mm Beretta and began the search, starting from the top floor, as he reminded himself of training protocols and worked to “ramp myself down.” In the basement, he saw an open door that led outside and became more cautious.

“I then made use of my handgun to tip the balance of power in my favor,” Crepeau, a resident of Missoula since 1976, wrote in his letter.

Honestly, that’s what having a firearm is all about; it’s about being able to tip the balance of power in your favor against those who are trying to take advantage of you.

For Crepeau, it was a home invader. For others, it may be a rapist or a mugger.

Someday, it may be a tyrannical government.

Criminals are often armed. They have the balance of power tipped in their favor. That is until they come across an armed citizen. Since criminals don’t want a fair fight, that’s enough to end the encounter. Sometimes, it’s not.

But at the end of the day, we can do all the math in the universe, but if the bad guys are going to be armed–and there’s no way anyone can honestly promise you that they won’t be–then you need to be armed as well. Plain and simple. Otherwise, the power is clearly in their hands.

I’m very glad that Crepeau was armed and able to protect himself. I want every law-abiding citizen who wishes to be armed to be so. If criminals knew that every other house had armed citizens, they’d turn to something else to make a living. While not everyone will have a gun since many people don’t want a gun, they’ll still benefit from a large number of armed citizens. Think of it as herd immunity from criminals.

Crepeau sent a strong message.

If that happens enough, it won’t need to happen at all.