Building some kits (and watching PT 109) this comes to mind:
What is the smallest useful survival kit?

Depends on the circumstance you're training for, but I'm going with "a PLB."

With that out of the way, I like Doug's PSP+ along with a Heatsheets 2-person blanket. With those two things and the right clothes and footwear I can do a lot with my modest skillset, assuming that I'm in North America and not in the water.

By preference I also carry a larger knife than is included in the PSP+, water and a vessel in which to boil same.

+++

The smallest kit doesn’t look like a kit, it’s called pocket carry. Knife, LED flashlight, Bic, et al ... a RESQLINK PLB will fit in your pocket too.

Doug's kit, with modifications, mostly for water procurement and purification. I have played with many, and these components works, are durable, it is small enough to carry with you all of the time (unless you are in a Speedo in a fragrance commercial) and it has what you need. the only thing it lacks is water procurement and purification. For that I like Aqua pouch plus . It is small enough and has a water container, strainer and purification tablets.

I have played with building the smallest possible, but still having the absolute necessities. What I have found is that, with training, and small knife, small saw, cordage (and dental floss works surprisingly well) and a lighter, you can do much. There are many variables, such as climate, weather, environment and situation (what are you doing there, what happened to put you into a survival situation), but generally these essentials will get you through. Knowledge does not take up room, and does not weigh anything. You always have your brain (or most people do, anyway), and if you prepare that thoroughly, you can make it in most situations. Every tool you add to that increases your chances of survival.

Now, I do have my favorite components to a kit, and there are some that I would try very hard to make sure I take with me if I can. UCO stormproof matches are one of those things, along with 1 or 2 charcoal grill starters ( Ignito ), but cotton balls and petroleum jelly packs very small and works well also. There are several small ferrocerrium rod fire starters, but my favorite I can't find anymore. A disposable lighter is great and small. There are a few small signal mirrors also.

I can go on and on. But then the kit gets big. You have to do the assessment of where you are going and what you can carry.

I am content to have a kit that is just reasonable in size and weight, but if I were trying for the smallest possible, I would probably include a Mora Light My Fire knife, and then some sort of metal container for melting snow and boiling water.

So much depends on the environment and the situation. A good question might be - What is included in a survival kit which weight one pound (or one half kilo)?

I completely understand that one can be separated from one's pack and supplies and that it's wise to have some gear in pockets or otherwise secured to oneself. That said I'm not a minimalist when it comes to survival gear. As an avid outdoorsperson with a keen interest in survival and bushcraft I can say that I have decent skills and practice a lot of them frequently. But survival isn't camping nor practice. So I endeavor to carry enough stuff on me to give me every possible chance at success.

Let me get this off my chest now.....nobody carries a PLB when the "smallest" survival kit will be both carried and used. If you are carrying a PLB then you are equipping yourself with a backpack (or other) and planning on spending time outdoors. "What if you are separated from your backpack...?" well then you are in all likelihood separated from your PLB too.

Ok....smallest survival kit. I personally take Doug's kit and add whirl bags (breast milk) and potable aqua tabs, plus some hurricane matches, squeeze light, and a heat sheet and put them in a aloksack bag. That is paired with my always on me EDC (knife, lighter, flashlight) and will be carried on-body when necessary, or stuffed in a briefcase/backpack, or tossed in a bag while traveling.... This adds some water storage and purification and extra fire starting. I almost always have a firesteel with striker plus small pill container of cotton balls with vasiline on me as one of these always is in any of my go-to jackets.

This plus knowledge should work for most of N. America. Knowledge trumps all.

BTW, Cody Lundin's first line kit is based around a Mora wrapped with other stuff and worn as a neck knife (per his 98.6 degrees book)

You don't need a survival kit in order to survive a trying episode - there are plenty of instances where non-kitted people pulled through, armed only with knowledge, ingenuity, and the will to survive.

How many people, well equipped with 'survival" items, failed to pull through? I don't have an exact number, but I am sure there are plenty of instances...

Knowledge and skills (plus a bit of good fortune!) can be of more value than gadgets in the challenge of surviving. But I will still carry survival items with me.