Two Is One, and One Is None

“Two is one, and one is none.”

This is a common prepper rule, but it also works well for ordinary non-prepper folks who move around a lot. I learned it years ago when I was much more into camping than I am now. Let's take a look.

It's a deceptively simple rule, almost Zen in its phrasing, but it holds deep practical wisdom. The idea is this: anything you truly need, you need a backup for. And if you only have one of something critical, you’re effectively living on borrowed time. Because when—not if—that thing fails, gets lost, breaks, or runs out, you’re left with none.

This rule of thumb applies to more than just survivalists or people heading into the backwoods. It’s just as relevant if you’re preparing for power outages, economic instability, or even just trying to make your daily systems more resilient. It's that last that applies to me. Probably to you too.

Take something as mundane as a flashlight. You’re out hiking or the power goes out at night, and you only brought one. The bulb dies or the battery goes flat — now you’re blind. But if you brought two, you’re still in business. Same for knives, lighters, radios, batteries, socks — heck, even glasses or toothbrushes if you’re traveling. The number of critical-use items that benefit from redundancy is surprisingly long.

I am not a prepper, with the exception of small things like an earthquake emergency bag, but I still use this rule in many ways. I apply this idea more broadly now, not just to camping or prepping gear, but to work. I have two notebooks where I write down important ideas. I back up my data not once, but in multiple ways—iCloud, external SSD, and even some notes printed to paper. I've duplicated financial systems, and stored system backups in different places. In other words, I try to never rely on just one of anything that matters.

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Jumping back to the notebook thing — that is really where I use this idea most in my daily life. Answer me this: how many times have you gathered up your work materials to go work at a cafe, but then as soon as you get there you realize you forgot something somewhat important? I'm going to guess it's happened more than once to you. You get there and you realize you forgot a pen, paper, your laptop mouse, your headphones, and so on.

Of course, this philosophy has its limits. You can’t carry three of everything or you’ll end up like a turtle flipped on its shell. You have to balance redundancy with practicality. Prioritize redundancy for mission-critical tools — the things that, if they fail, derail your entire plan or compromise your safety.

Here's how I use this on a day-to-day basis:

I find myself moving around a lot. I visit companies a lot to teach or lecture, student homes, and I routinely stop in at the library or cafe in my travels to work on the go. All these things in mind, I need some common work things with me all the time. This involves notebooks, pens (fountain pens, of course), headphones for working in a cafe, a computer mouse, a water bottle. Some other stuff too, but that will give you some idea.

As much as possible, I have everything doubled so that I never have to take it out of my work bag. I have a notebook that always lives in my bag, for instance. I have a fountain pen pouch that always lives in the bag — I do rotate the pens as I need to refill ink, but I am careful to always add one if I take one away and always leave the pouch in the bag. I try to always carry my AirPods with me, but if I happen to forget them, there is an old backup pair of headphones that live in my work bag.

The key way you can see that I'm applying this rule is to always keep my second copy of things in my work bag so that I eliminate or greatly reduce the negative consequence of forgetting something.

However you apply this old prepper rule, it is definitely worth always keeping in mind.

Because two is one—and one is none.