Evolution optimized our brains with certain cortex-hardwired faculties that quicken learning and ease decision making. While no one has yet claimed to have definitively isolated them all, Steven Pinker used his encyclopedic knowledge of the state of the sciences of psychology and neuroscience to put a stake in the ground in his 2002 book The Blank Slate. Here are the eleven core faculties that he identifies.

  1. Simple Physics: (We intuitively know that...) Objects persist in space and time. When they fall, bounce, and bend, they follow consistent laws of motions and force.
  2. Simple Biology: All living things possess an invisible essence that gives them their power, drives their growth, and is inherited by their progeny. A dead thing no longer possesses this invisible essence.
  3. Simple Engineering: Certain objects made by other people can be used to ease work. Recognizing the functions of these objects and the intentions of the designer is easy.
  4. Simple Psychology: Other people are similar to you in that they possess thoughts and motives which drive their actions. (Called the theory of mind.)
  5. Spatial sense: We know where we are in relation to other things. We know where to move for greater protection from or access to something.
  6. Number sense: Small numbers of objects are easy to quantify. Larger groups of objects are easy to compare.
  7. Simple Probability: We can predict the likelihood of certain events based on intuition and past experience.
  8. Simple Economics: We have an innate sense of fairness in the exchange of goods and favors.
  9. Mental database & logic: We can generalize knowledge, apply knowledge, and create new hypotheses with operators such as AND, OR, NOT, ALL, SOME, NECESSARY, POSSIBLE, and CAUSE.
  10. Language: We easily adopt spoken language that enables us to share our mental database and logic. This includes a memorized vocabulary, rulesets for combining these words into sentences, and memorized exceptions.
  11. Simple Morality: Certain people, objects and land are worth our protection and respect. (Called our moral circle.) People, objects, and land outside of this group may be subjugated.

He also identified two other faculties worth mentioning, but they are probably better categorized as emotions as they are driven more by the amygdale rather than the cortex. Still you can imagine someone saying, "Ain't you got the common sense to...?"

  • Fear: We can quickly and fairly accurately assess danger.
  • Disgust: We can intuitively assess contamination in people and objects.

This collection of faculties is, if anything, what can be called common sense. (It even fits Sir W. Hamilton's definition, "The complement of those cognitions or convictions which we receive from nature, which all men possess in common, and by which they test the truth of knowledge and the morality of actions.") No one has to teach kids not to run into trees, or that one kitten is less than two, or follow peoples' gazes to see what interests them. It's ingrained. Common.

It will probably be a while before neuroscience or psychology can authoritatively confirm or refute Pinker's list. But presuming he's close, so what?

I work in the field of Human-Computer Interaction, so it's quite useful for me, when I'm building interfaces between processors and brains, to understand what people are naturally good at, what faculties they may bring to bear to a given task, and what instincts guide their guesses when things go wrong.

But even people outside of my field can benefit from this awareness, because these instincts can hinder life in the modern world. How can this be? If evolution built them (and they sound pretty good to me) aren't they reliable? Well, not entirely. These common senses are very functional in the Pleistocene nomad's world of illiteracy, woolly mammoths, and a carrying-capacity limit to possessions, but we don't live in that world. (The gene may be selfish, but it's damned slow.) The world we live in has new, unintuitive areas of knowledge we must grapple with and edge cases where these instincts are wrong.

Unintuitive Knowledge

We can bring some of common sense to bear for the first problem of vital but unintuitive knowledge. That's what we're doing when we teach children to read and write using their natural mastery of language. It's why your mathematics teacher kept telling you to draw out word problems, to get you to shift from tough abstract reasoning to the easier task of working with spatial relationships. We hijack our simple engineering sense to understand the details of complex biology, as we comprehend each organ as a widget with a function. Generalizing this, when you're faced with a tough problem in an unfamiliar field, change your approach to the problem through one of your common senses, and you'll probably be more effective. Similarly, when you're teaching a complex subject, build on your student's common senses, and you'll probably have more luck.

Edge Cases

More critically IMHO, as science pushes our understanding of the universe and our selves, we are confronted with new complexities and edge cases where these instincts are actually dysfunctional, or wrong. Sometimes this is a matter of scale, as when particle physics confounds our Newtonianish instincts. But often this occurs where a sense is over-applied. For instance, the concept of a soul derives from our biological sense and our theory of mind. It worked pretty well for telling whether someone or something was either dead or alive. But modern science is presenting us with questions for which the concept of the soul does harm. For instance, this concept led ideologues in the Roman Catholic Church to outlaw condoms, even at the expense of overpopulation, unplanned parenthood, and devastating sexually transmitted diseases. Recognizing such dysfunctional sense will become even more important as the world plunges further ahead into the unfamiliar world of biotech.

Similarly, our common sense for simple engineering causes lots of cognitive dissonance for people struggling to reconcile evolution with their intuition that design implies a designer, and so we get Scopes Monkey Trials and Mississippi textbook warning labels.

None of this means we should abandon common sense (if we could). It is generally a useful thing. What it does mean is that we should recognize our common sense for its origins and limitations, and keep a skeptical head about it. Common doesn't necessarily mean good.