When it comes to protecting your life, common sense is one of your greatest weapons. Quite often, common sense is called "uncommon" or some other such phrase to show how little used it is. I agree, but with a caveat. Common sense isn't some universal bank of knowledge, untapped by the masses. Common sense is simply the set of logical, straightforward conclusions that can be derived from a given set of facts. For example, common sense might say that it is a good idea to look both ways before you cross a road. That is based on the fact that cars travel very quickly, and getting hit by one would be very bad. This particular one is quite obvious, and this is why it is called common sense. It should be obvious.
What seems to be most lacking are the facts that these inferences are based on. For instance, most children have to be told to look both ways before crossing the road. There is no reason to believe that these kids lack any survival instincts, but rather they are not aware of the basic facts. Whether the child understands the "common sense" argument for looking both ways depends on his or her understanding of "getting hit by a car would be bad." Certainly, when a child understands that it is a serious thing, he or she will look both ways. I am obviously over-simplifying here, though. Even adults forget to look both ways, but it doesn't mean that if they thought about the particulars, they wouldn't have looked.
Something like looking both ways before crossing a road actually ties in pretty quickly with martial arts (I knew you were thinking that). All forms of martial arts teach self-defense at their core. The term "martial art" literally means military art. People were originally taught martial arts to fight enemies in combat, and while dealing death was a primary focus, defense was important too. Today, the martial arts encompass a huge number of disciplines, but most of them still use self-defense as one of their primary "selling points". Some styles have evolved into a sport or as exercise only (more on this later), but the martial arts are still regarded as a way to defend oneself against attack. Self-preservation doesn't always have to be about keeping your opponents sword from penetrating your body, though. Looking both ways before crossing a road is self-preservation too.
There are a lot of things in martial arts (e.g. self-defense) that are obvious when you think about it, but most of the time people lack the facts to make those conclusions. In this section, I'll go through a few examples, and provide a framework for how critical thinking and common sense can help in self-defense situations.