Common sense

Not only should we not be surprised that many aspects of reality are counterintuitive, we should expect it to be counterintuitive. Any account that conveniently dovetails with our common sense should be met with skepticism.John Locke invented common sense, and only Englishmen have had it ever since! Common sense is something which you think you know to be true but that may not actually be true. It is a way of reasoning based on heuristics, and basic rationality applied to that knowledge. However, the danger with common sense is that the scope of knowledge can be quite wrong, and the basic rationality can simply not be deep enough for an attempt at finding truth. For something that is supposed to be common, however, sometimes it would seem like it should be called rare sense.

Uncommon nonsense
Common sense is not what the mind cleared of cant spontaneously apprehends; it is what the mind filled with presuppositions… concludes. For all practical purposes, "common sense" is a body of 'rules of thumb' for how the world operates, which may or may not be shared with others in the community. For the most part, this works fine; for instance, it is common sense not to walk in front of a speeding car, since, at some point in your past, you've seen fast things hit fleshy things and said to yourself, "ouch." Common sense would also suggest that a small person might best avoid antagonizing a large person as he can easily imagine the effects of flying fists. Common sense is not taking the time to mentally calculate the effects of the car's momentum being transferred to your own body, how far the impact will push you, or any other higher-end science-based function. Get out of the damn way, 'cause that's bound to hurt! will suffice. This is all part of our survival mechanisms to make it more likely that we live on to reproduce.

It's not entirely impossible for people to have their common sense overridden by logical deduction if it's explained properly. While most people get the or the  completely wrong because their common sense (that is, the inferences that we develop over time) comes to incorrect conclusions, pretty much everyone accepts the right solution when it's described to them.

The lesson: don't abuse your common sense. Sometimes it is best to get your science on instead of thinking shallowly.

Science and common sense
Common sense is nothing more than a deposit of prejudices laid down by the mind before you reach eighteen.

Common sense plays a role in science. If there are two possibilities that could both be true, it is accepted by science that the most simple, most "common sense" answer is the place to start, and until or unless it is disproved or "wobbly," it should be given more weight than an answer that breaks common sense until more evidence can be gathered. Mostly this is settled by Occam's Razor, in which case the preferred answer is the one that requires the fewest assumptions.

Often, many physical laws that are true defy common sense. Quantum mechanics is full of this sort of thing. How, exactly, does an electron need to turn through 720 degrees to come back to where it started‽ Or how can an electron pass through a node in the wavefunction if it can never actually be at said node in the first place? The fact that the evidence defies such common sense is in fact a massive barrier to people trying to fully understand quantum theory — and numerous interpretations have been produced to try and make it less nonsensical.

But you don't need to go into exotic theories that were only developed once technology had grown to the point where we could detect it. For example, it is common sense that if you drop a ball, it will fall. It always has, and it always will. What is not such common sense is that gravity is a force (maybe) or an interaction between the ball and the Earth causing the ball to be drawn down to the Earth, and the Earth to in fact be drawn up, theoretically. Common sense would in fact deny that possibility, though it is the more true one.

Common sense would also tell us that heavier objects drop faster than light ones — but this is actually the result of air resistance rather than mass, as was proven by science and then aptly demonstrated on the moon centuries later. In fact, the inferences we make about such objects can bleed into otherwise obvious questions, such as asking which is heavier, a pound of bricks or a pound of feathers — people might instinctively think of the bricks before having that "Wait a sec!" moment.

So if it's wrong, what's the point?
Common sense allows us to function in the everyday. Common sense tells us that the sun came up yesterday and will tomorrow, so we do not have to go to bed in fear. Common sense tells us that eating too much will make us sick (since it has in the past), and that red glowing things tend to be hot, and red signs tend to indicate danger. It is our very survival mechanism at work, even if it sometimes lies to us.

Common sense is, in fact, our very entry into becoming a thinking thing. "Does this new idea I have fit into the everyday world as I know it? — and if not, why not?"

Flawed use
The truth may be puzzling. It may take some work to grapple with. It may be counter-intuitive. It may contradict deeply held prejudices. It may not be consonant with what we desperately want to be true. But our preferences do not determine what's true. An appeal to intuition is an argument that because a proposition does not match one's experience of how things work in general, or how we believe they should work, then that proposition is not true.

Intuition is based on our prior experience and understandings. An appeal to intuition is essentially an argument that for something to be true, it must be similar to what one already believes to be true.

The history of science is replete with evidence that appeals to intuition do not reliably yield truth. Modern science has shown that virtually every time we examine some part of the world distant from our immediate experience, we discover that some of our intuitions do not match reality. For example, our intuition says that: And so on.
 * Earth is flat and fixed in space, since we do not notice any curvature from most perspectives nor does Earth have to move in space for us to observe other celestial bodies moving through its sky, but it turns out that the surface of Earth does curve over the totality of its diameter and Earth is moving in space, albeit in a way we never notice, and that is how we get days and nights and the (astronomical) seasons (an Earth that was fixed in space would still have weather).
 * it doesn't make sense:
 * for something to be both a wave and a particle, but it turns out this is true if you look at things far smaller than what we have direct experience with.
 * that there could be an upper limit on how fast anything could possibly go, but if you look at velocities far outside the range of what we have direct experience of, this turns out to be true.
 * nothing can live in boiling water, since the most obvious life forms we are familiar with cannot, but it turns out that thermophiles can and do.
 * continents do not move, since we do not notice any such movement, but it turns out that they do in ways that have produced enormous movements over geologic time (a time scale much longer than the seventy or so years that a person tends to live).

In the case of evolution, our intuition may say that seemingly random processes can't result in highly ordered systems &mdash; but we have no direct experience of time scales of hundreds of millions of years, and it turns out that our intuition about what can happen in an hour or a year isn't accurate when we apply it to far longer time scales (or even when we apply it to time scales that short, for that matter).

This fallacy is related to but distinct from an argument from ignorance.

Confusion
Common sense is often confused with rational thought, in that people often believe common sense must be true and act incredulously to rational or scientific ideals that contradict common sense. This is because the human brain can easily work with ideas like common sense and rules of thumb but can't quite cope with physics and statistics. Common sense is often made up of much prejudice and snap judgement, and therefore is not always useful and can certainly be irrational even when it is useful.

A similar train of thought is used by religious fundamentalists who insist that while other religious groups may have their own, incorrect, "interpretation" of holy scripture, their own group does not have an "interpretation" at all. They, you see, simply read the plain and direct words of scripture, whose meaning is clear to anybody with common sense. And one can use "common sense" to gloss over any difficulties thrown up by biblical literalism too.

Under no circumstance ever confuse "common sense" with common law. You'd be surprised how many people get those two mixed up, and how many of them wind up going to jail. "Common sense" does appear a lot in the common law, but only in places where you might expect it (and often couched in the terms of the "reasonable person" rather than "common sense"), because the law is more likely to deal with things like cars hitting people and less likely to deal with things like particles that are also waves.

Oh, and if you want to use "common sense" as an adjective, then the word is commonsense.