We can agree that something is common sense as long as we share the same set of assumptions. I think people really don't understand how much of a problem this is, because they assume that what they think is common sense is right.

Someone was just telling me the other day that she was mystified at how we couldn't just use common sense to make policy. It seemed completely ridiculous to her—why isn't there someone in politics who could say that it's common sense that everyone should have equal access to healthcare? But that's a belief that is deeply disputed! And people who disagree with her also feel that they are using common sense.

So common sense is actually very good for resolving everyday situations, where everyone shares the same set of assumptions. The problem is that it feels so effective to us in these circumstances that we're tempted to use it to make decisions and plans and predictions about situations that are not everyday situations.

What it all comes down to is that pretty much everything seems obvious to us when we believe we know the answer, and so we make simple assumptions about complex situations. This causes us to disagree with one another over things we haven't really thought through as much as we may think we have. While we're wired to make these kind of assumptions, we can keep ourselves from getting too attached to them by keeping in mind that even our most valued beliefs could be wrong. This is just one more good reason to always keep an open mind.