If I couldn't bring a knife or my brain along...........most of the time that I would potentially be in such a fix, I would be in the Maine woods. I'd bring a reliable lighter along. Or some other means of starting a fire. But any lighter that uses a flint would at least make a spark.

A fire can provide warmth and maybe be used as a signal.

Maybe I'd bring a gun, instead, so I could shoot myself for being out and about without a knife and a PSK.