I have tended to emphasize common sense as a basic source in attempting to philosophize or otherwise understand reality. Let me explain what I mean by the idea of common sense.

The basic idea is that something is common sense when everyone agrees that something is true. If we start with this vague account, something will be more definitively common sense to the degree that it is truer that everyone agrees, and likewise to the degree that it is truer that everyone agrees.

If we consider anything that one might think of as a philosophical view, we will find at least a few people who disagree, at least verbally, with the claim. But we may be able to find some that virtually everyone agrees with. These pertain more to common sense than things that fewer people agree with. Likewise, if we consider everyday claims rather than philosophical ones, we will probably be able to find things that everyone agrees with apart from some very localized contexts. These pertain even more to common sense. Likewise, if everyone has always agreed with something both in the past and present, that pertains more to common sense than something that everyone agrees with in the present, but where some have disagreed in the past.

It will be truer that everyone agrees in various ways: if everyone is very certain of something, that pertains more to common sense than something people are less certain about. If some people express disagreement with a view, but everyone’s revealed preferences or beliefs indicate agreement, that can be said to pertain to common sense to some degree, but not so much as where verbal affirmations and revealed preferences and beliefs are aligned.

Naturally, all of this is a question of vague boundaries: opinions are more or less a matter of common sense. We cannot sort them into two clear categories of “common sense” and “not common sense.” Nonetheless, we would want to base our arguments, as much as possible, on things that are more squarely matters of common sense.

We can raise two questions about this. First, is it even possible? Second, why do it?

One might object that the proposal is impossible. For no one can really reason except from their own opinions. Otherwise, one might be formulating a chain of argument, but it is not one’s own argument or one’s own conclusion. But this objection is easily answered. In the first place, if everyone agrees on something, you probably agree yourself, and so reasoning from common sense will still be reasoning from your own opinions. Second, if you don’t personally agree, since belief is voluntary, you are capable of agreeing if you choose, and you probably should, for reasons which will be explained in answering the second question.

Nonetheless, the objection is a reasonable place to point out one additional qualification. “Everyone agrees with this” is itself a personal point of view that someone holds, and no one is infallible even with respect to this. So you might think that everyone agrees, while in fact they do not. But this simply means that you have no choice but to do the best you can in determining what is or what is not common sense. Of course you can be mistaken about this, as you can about anything.

Why argue from common sense? I will make two points, a practical one and a theoretical one. The practical point is that if your arguments are public, as for example this blog, rather than written down in a private journal, then you presumably want people to read them and to gain from them in some way. The more you begin from common sense, the more profitable your thoughts will be in this respect. More people will be able to gain from your thoughts and arguments if more people agree with the starting points.

There is also a theoretical point. Consider the statement, “The truth of a statement never makes a person more likely to utter it.” If this statement were true, no one could ever utter it on account of its truth, but only for other reasons. So it is not something that a seeker of truth would ever say. On the other hand, there can be no doubt that the falsehood of some statements, on some occasions, makes those statements more likely to be affirmed by some people. Nonetheless, the nature of language demands that people have an overall tendency, most of the time and in most situations, to speak the truth. We would not be able to learn the meaning of a word without it being applied accurately, most of the time, to the thing that it means. In fact, if everyone was always uttering falsehoods, we would simply learn that “is” means “is not,” and that “is not,” means “is,” and the supposed falsehoods would not be false in the language that we would acquire.

It follows that greater agreement that something is true, other things being equal, implies that the thing is more likely to be actually true. Stones have a tendency to fall down: so if we find a great collection of stones, the collection is more likely to be down at the bottom of a cliff rather than perched precisely on the tip of a mountain. Likewise, people have a tendency to utter the truth, so a great collection of agreement suggests something true rather than something false.

Of course, this argument depends on “other things being equal,” which is not always the case. It is possible that most people agree on something, but you are reasonably convinced that they are mistaken, for other reasons. But if this is the case, your arguments should depend on things that they would agree with even more strongly than they agree with the opposite of your conclusion. In other words, it should be based on things which pertain even more to common sense. Suppose it does not: ultimately the very starting point of your argument is something that everyone else agrees is false. This will probably be an evident insanity from the beginning, but let us suppose that you find it reasonable. In this case, Robin Hanson’s result discussed here implies that you must be convinced that you were created in very special circumstances which would guarantee that you would be right, even though no one else was created in these circumstances. There is of course no basis for such a conviction. And our ability to modify our priors, discussed there, implies that the reasonable behavior is to choose to agree with the priors of common sense, if we find our natural priors departing from them, except in cases where the disagreement is caused by agreement with even stronger priors of common sense. Thus for example in this post I gave reasons for disagreeing with our natural prior on the question, “Is this person lying or otherwise deceived?” in some cases. But this was based on mathematical arguments that are even more convincing than that natural prior.