There's a lot of stuff I like to have on me, if I can: rope, batteries, flashlight, masking tape, sharpies, a bottle opener, bandages, a multi-tool, a flash drive, a needle and thread, a microSD adapter, lockpicks, an MRE... Mostly stuff I don't carry anymore.[1. I still generally have the rope and the microSD adapter.] It just all gets a little heavy. But there are three things that I would ideally always carry -- two of which I don't have right now -- because they're light, useful, and philosophically significant: a knife, a lighter, and a pen.
I'm a big fan of the worldview that the most significant thing about being human is the use of tools, and to me, these three items represent the three major categories of human achievement that I can identify.
The knife is the most basic. It represents the simple manipulation of raw materials into a useful form: creating value by changing the shape of things. Pocket knives are pretty complicated, but the basic idea -- a blade -- is about as simple as you can get when you're looking for valuable tools that aren't naturally occurring.
The lighter represents fire, obviously, but more broadly, it represents a more fundamental chemical control of things than the knife. With fire, we were able to start cooking food, which sped up digestion and gave us time and energy to do the things that built organized social structures. We're able to alter the atmosphere, making it hotter or drier. It's also got the most potential for violent, large-scale destruction, which has been generally true of chemistry for the rest of human history following the use of fire.
The pen, the one I still carry all the time, because it's the most useful, represents language, and more broadly, the complex social structures that language enables. Language is super-important to me -- that might be obvious, because I have a blog and my career plan is to try and get people to pay me for the stuff I write -- but it's one of those things that literally no-one[2. Okay, not literally. I'm sure that if you searched for a little while you could find some obscure fringe cases of people who don't use any language at all. But, like, can you name anyone you know, personally, who doesn't use language? Anyone you've met?] can do without. You'd have to be in absurdly particular conditions to get through an entire day without using any language -- not speaking, but also not hearing-and-understanding, reading or writing, interpreting symbolic images or gestures, and so on. It would be far more difficult than getting through a day without using any physical tools -- I'm sure a lot of people can make it through a single day, naked in the woods -- or without instigating any chemical reactions -- for that one, you can spend the day fully dressed, in a dark shed.
I don't have a knife right now because I lost the one I liked, and none of my other knives are quite worth carrying around in situations where they could be used as leverage if someone wanted me punished for something. Instead I just keep them in my bathroom at home, for hair cuts.
I don't have a lighter because I misplaced it, but it shouldn't be difficult to find if I decide to start carrying it again soon. At least a few times a month, I end up missing one or the other of them, because they really are incredibly useful.
But I've never gotten through a day without a pen where I didn't feel like kicking myself for not bringing one. I often end up regretting leaving a room without taking a pen with me. And, obviously, I'm usually carrying two or three notebooks, as well. I'm not sure, exactly, where they should fit on the list.