Common Sense

On the Uses and Limitations of Common Sense and Intuition

Often arguments about personal identity begin or end with an appeal to common sense or intuition, i.e., a conclusion that is obviously true. To some, it may be obvious common sense that two copies of a person are two different people; to others, it is just as obvious that they are the same person. Yet that doesn’t mean common sense is never useful; on the contrary, any good theory of personal identity should match our common-sense conclusions, in certain scenarios. Exploring the origins of common sense, when it is useful, and why it sometimes fails to serve us is the topic of this essay.

What we call common sense, intuition, or “obvious” is really just a label for conclusions we make without logical thought, using the pattern-matching part of our brains. (Which is to say, the vast majority of our brains; logical thought is a thin and often shaky layer on top this ancient and powerful pattern-matching machinery.) Confronted with some scenario, we quickly and unconsciously compare it to other, similar scenarios we have faced throughout our lives, and produce a conclusion based on what has worked in the past.

There is nothing wrong with this approach in general. The recent stunning success of deep neural networks over the logical, symbolic approach to AI has shown how powerful and successful pattern-matching can be; we would never get through our day if we had to logically reason out every moment-to-moment decision. But a key limitation of this approach – both with AIs, and with our own brains – is that it only works for situations similar to the ones on which the network was trained. In other words:

Common sense only works for common situations.

As applied to personal identity, common sense is really good at addressing issues with which we are all familiar:

• After sleeing through the night, am I the same person who went to bed the night before?

• Are twin brothers the same person, or different people?

• Are you the same person after suffering physical injury?

Even if the common-sense answer is nuanced (as it probably should be), it will provide a great starting point to constrain on any theory of personal identity. In other words, if a theory of personal identity disagrees with common sense in a common situation like the above, then it’s probably a lousy theory.

Of course even in common situations, one must always be careful about cultural bias and limited information. At one point it seemed obvious that the Earth was stationary, and that the Sun, Moon, and stars all moved around it. Yet that common-sense position was dead wrong, as science eventually showed. Or take the example of slavery, once a widely accepted part of society in many parts of the world, and now universally seen as abhorrent (largely due to developments in philosophy). So common sense can certainly give wrong answers, even on topics where people have direct experience, because in many cases it is essentially a distillation of cultural norms. All that said, it’s still generally good at predicting what will happen to a ball when dropped, whether you will feel tired after a long run, and whether, after going to sleep, you will wake up as essentially the same person you were the night before.

But common sense is terrible at tackling situations we have never actually encountered, like these:

• If you go back in time and meet yourself, which one is the real you?

• If I find out that aliens visit my bedroom each night, disassemble me atom by atom, and put me back together from different atoms, how should I feel about that?

• If you and I wake up and find that we have magically swapped bodies, which one is “me” and which is “you”?

• If a Star Trek transporter malfunctions and simultaneously beams a crewman to two different locations at once, which is the real one?

• Conversely: if a different transporter accident combines Tuvok and Neelix into one person (Tuvix), is the captain justified in forcibly separating Tuvix against his wishes?

These are what’s known in AI circles as “out of context” questions — they are far outside the domain of experience on which our brains were trained. It’s possible, if you grew up reading a lot of science fiction, that your common sense may be slightly better equipped than most to deal with some of these; fictional experience can substitute for real experience to some degree. But even in that case, any “intuitive” or “obvious” answers should be treated with suspicion.

This is why people can come to such different conclusions about the nature of personal identity, particularly when any sort of duplication is involved. We have never, in all of human history, had the ability to duplicate a person; nor to separate a person from their body in any way. So our brains have basically learned to strongly associate a person with their body, or with a nebulous mind/spirit that we imagine being carried around by the body (and which we also imagine to be indivisible and unreplicable). And so when faced with a scenario like the above, our experience does not apply, and common sense is likely to generate answers that are not logically consistent.

In such cases, then, what is one to do? This is where the quick, “obvious” answer must be rejected, or at least held in probation, and the tools of logical reasoning must be applied. This is indeed the whole business of philosophy: to start with some minimal set of premises, and follow the logical implications to useful conclusions about how we ought to think about some question or issue. It’s a similar endeavor to mathematics, where practitioners began with humble observations (like “things can be counted”) and developed all manner of useful but often surprising constructs, like calculus and negative numbers. Philosophy, while not quite having the rigor of mathematics, nonetheless strives to do the same thing for issues like ethics, morality, and the nature of personal identity.

Much of that work comes from advancing a theory or position, and then testing it in various ways. The next essay will delve into this in more detail, but common sense can certainly serve as one of these tests. A theory which, for example, results in concluding that you are in no way the same person after a nice nap would clearly be lacking, and the quest for a useful, coherent theory of personal identity would have to look elsewhere. But such common-sense tests must be applied only in common scenarios, and only where we are sure cultural bias is not blinding us to deeper truths. In any more esoteric scenario, where we must rely on imagination rather than experience, our common-sense intuition must be given little to no weight.