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What is common sense? It is a widely used phrase that has a vague meaning. It is generally taken to mean a way of thinking or judging that is so elementary that almost anyone can be expected to follow it. It's not quite the same as 'obvious', but is expected to be so easily reproduced that it requires no deep justification.
Some think of common sense as simple logic. "If you don't want someone to steal your car, you shouldn't leave the keys in the ignition. It's just common sense." The analysis is very simple. If you don't want something to happen, and doing something makes it more likely to happen, then you shouldn't do it. This is straightforward, deductive logic. And while people may not have taken classes on logic or studied it in any detail, it is widely expected that people will understand this.
Logic is only part of the equation, though. It also assumes a common set of knowledge. The same kind of simple, logical step may be used in an advanced field of science, but it wouldn't be described as common sense. For it to be common sense, it really must be common.
While common sense is often treated with some amount of respect, it is also recognized as limited or flawed. Elementary reasoning can lead to mistakes in a number of ways.
One of the classic stories of science is how people thought that heavier objects would fall faster than light objects. An object twice as heavy as another was expected to fall twice as fast. This was a case of common sense leading to an incorrect conclusion. It seemed right. If the heavier object was pulling downward twice as much as the other, shouldn't it fall twice as fast? But in fact, they fall at the same rate (except with very light objects that experience more significant air resistance).
Common sense can break down because it relies on many unmentioned assumptions and a body of knowledge that might be flawed. Some people believe that poverty causes crime, and that it make sense. If you don't have much, you will be tempted to take it from others. But the relationship is usually the other way around, where crime causes poverty. If you spend your time trying to take wealth from others, you won't put in the time and effort at being productive.
When it comes to philosophy, common sense has a mixed record. Often people's common sense view of the world is more accurate than philosopher's who are tempted to deny the power of reason or existence itself because they lack sufficient answers to hard questions. A common sense approach leaves the foundation unstated, but is able to appreciate the consequences of reason and science and recognize that they must work well to produce the results that they do.
There's one area where common sense doesn't work well at all. This is the field of morality. Common sense relies on an existing foundation of knowledge and assumptions. A person's morality is part of this foundation. If you are raised in an altruistic world, altruism will be part of your basic assumptions about what morality is and how you should act accordingly.
A morality of self-interest, in contrast, would be completely alien to someone raised in an altruistic morality. None of its conclusions would "make sense". The altruist would say to himself, "I know that helping others is moral. It's the definition of good. And I know that selfishness is the cause of all the evils in the world. And here is someone telling me to be selfish! That's obviously wrong!".
Appeals to common sense don't make sense when questioning a fundamental assumption. Common sense lives on a foundation of your existing beliefs. It takes them for granted and applies that information to slightly different circumstances. It's a method of extending what you know, but not a method of questioning what you know.
One way to think of common sense is a method of integration. You already have a base of knowledge you accept as true. When new ideas are presented, you can test to see if the new ideas are compatible with the old. This uses logic to test the integration. If the test fails, meaning the new idea is not compatible with the old, you say, "That doesn't make any sense." If the test passes, you say "That make sense". And if the test passes, and it seems the idea could be deduced from the old knowledge, you might say "That's just common sense".
This limits the usefulness of common sense. It never questions the initial assumptions or base of knowledge. It only applies that information by testing new ideas with it. If the initial foundation is wrong, common sense will lead to incorrect assumptions. And if you are challenging the very foundation of a person's belief, common sense can be the enemy of persuasion.
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