A base layer of light merino wool is indispensible, and will keep you warm while moving to surprisingly low temps (you do need a windbreaker of some sort if there is a wind). It keeps you warm even if wet. There is no substitute that’s even close. A polypropylene underlayer is pretty worthless if sweated into, even if fleeced, you get cold and can’t get warm – the sweat lays right on your skin and isn’t moved through the fabric.

A middle wool layer isn’t necessary in the summer when moving, usually. If not moving, or in the winter when moving a bit, it is very useful. A top layer needn’t be wool, should shed water, and should insulate when wet (polyester fluff fill is good). Your top layer will absorb water from sweat, and after a long day, your top layer will be wet, even if water proof. Youcan live for days in pretty good comfort in a wet wool coat. If you’re going to be in the rain all day, especially near freezing temps, a wool overlayer is best. Except you’ll end up carrying 20 lbs of water…

In consistently below freezing temps, down-filled is best, though you can sweat it through if you don’t take care to remove layers when you’re getting hot. I’ve never had down-fill fail to keep me warm, even when fairly wet, but I’ve not gone more than a couple hours after getting it wet.

Cotton is good only for fair weather.

I never suffered from the cold in the cascades after moving to a merino wool underlayer and staying with wool over it, with a good wind breaker. You can still die of hypothermia though, even in a sleeping bag in a tent, if you get exhausted and cold before climbing in. Carry a small stove w/fuel and soup – hot food will help, may save your life.