This thread subject comes up every once in a while, and I always learn a thing or two, and change my critical items contents accordingly. For example, I am going to add a square of riteintherain paper with my name, contact info, etc, into my kit. (So they can identify me when I melt out of the snowpack in Spring..)
I imagine the most epic and dire situations would start out with the loss of my pack due to avalanche, precipice, river, theft, or some situation I could not foresee.. If I haven't lost my pack and all its luxuries I would not call that situation 'survival'- I'd call it 'camping'.
So, if I did loose that pack, I'd want a few critical items on my person at all times. Currently I usually* carry a ziplock with: (Ritter kit*, micropur and motrin), a space blanket, leatherman S4, Fenix L0D, mini bic lighter, and a micro carabiner/lanyard to secure it all inside my pants cargo pocket. If I've lost my pants as well; I'm really in a world of hurt.
Its about 324grams, and to ensure it's not dead weight, I do not carry duplicates of these items. As much as possible, these are the items I would use on a daily basis in the field; the mini bic, S4, Fenix, sparklite, even the space blanket which many consider useless in a survival situation can be used as a ground sheet.
And if any readers think this topic is fringe, consider that some of the "sub-x pound" packing lists posted at this site are not really are not camping lists, they are minimal lists which provide for survival, and not a whole lot of comfort (and assuming mild weather). So considering what is a minimal survival list can get you closer to a FSO weight goal.
*usually = when more than whistle distance from a well travelled trail.