Over the years I have found that common sense is one of the rarest commodities in the world - particularly among the 'well educated' class

It's the ability to see things as they are, as well as their consequences, regarding everyday matters. There is mass agreement to "common sense."

For example, not letting small children handle knives, or wearing a warm coat in very cold weather is using common sense.

Fire is hot.. Water is wet.. Pain hurts.. That's common sense.. Self evident stuff I'd say.. Things all people should know.. Doesn't need to be figured out...

It can mean a lot of things depending on the person using the phrase and the subject. Usually it is most used by people who have it the least.

Sometimes it is just another cheap anti-intellectual shot, like "geek", "nerd" and "bluestockings", but if there is any thing called common sense which has value. Then I would say that it is the ability to make good value judgments about issues, where there is not time, freedom or resources for testing and fact checking, it is therefore a good last resort when we have nothing else.