Why Rational People Disagree

Doubt is not a pleasant condition, but certainty is a ridiculous one.
-- Voltaire

A few months ago a friend sent me an email, asking, in obvious frustration, how we could share so many common beliefs about the world yet come to such disparate conclusions about political issues. I think this was an excellent question, and well worth answering seriously.

Fundamentally, people disagree because most of their beliefs are supported by evidence that is at best fragmentary and at worst imaginary. To make the distinction between beliefs that are substantially justifiable and those that are not, let’s look at a simple example of each type. First, consider the following rough-and-ready proposition:

Water freezes at 32 degrees Fahrenheit.

Assuming we are talking about more-or-less pure water in typical conditions that prevail on the surface of the earth, and putting aside miscellaneous philosophical problems involving measurement, etc., this proposition is quite straightforward. We all know what water is, at least in a raw, experiential way. Most literate people are at least vaguely aware that the property they perceive as hot or cold is reliably measurable, and even if they aren’t familiar with the Fahrenheit scale they are familiar with the relationships between numbers. 32 is greater than 31.

The freezing water proposition can be put to the test any number of times and yield the same result within a very small margin of error. It is not a proposition about which normal, reasonable people can differ. There are no special sects of people that firmly believe that water freezes at 78 degrees Fahrenheit because their ancestors believed it, or because it was so written in their holy books. The freezing point of water is a testable, reliable, and comprehensible fact.

Sadly, most of our beliefs are far less straightforward than the one described above. Consider this proposition:

Doubling the length of jail sentences will substantially reduce the rate of criminal theft.

Let’s assume we are proposing this as a general rule – that we are making an assertion we expect would be applicable to all societies at all times. This claim would be extremely complex and difficult to prove.

In the case of the freezing water proposition, we were dealing with an inert physical substance – not with the convoluted causal logic of human beings. Considered as a physics problem, it is not especially difficult to understand how the behavior of water molecules causes a change of state from water to ice at a certain temperature. Having understood this, it doesn’t matter whether we are talking about a drop of water or an arctic lake. The same physical principle applies. Even if we do not understand the underlying physics, we can still take note of the monotonous reliability of the phenomenon of freezing.

The theft proposition, on the other hand, is staggering when considered as a physics problem. The number of factors that influence even individual behavior are almost without limit. Further, the precise influence of each factor can neither be readily calculated nor measured. It is doubtful whether the human brain could grasp the operations of even one other brain of equal complexity, except in either crude generalities or fragmentary details. To make matters worse, the number of factors that must influence the rate of criminal theft would have to be something like the huge (if unknown) number of factors influencing each potential thief, multiplied by the total number of potential thieves. Even specifying what information one would need to collect is daunting, let alone the question of knowing enough about causal relationships to process that information into any sort proof.

For the sake of argument though, let’s imagine we could solve the formidable problems of working out the exact physics of the brain -- and everything that might influence the brain. Let’s further assume we have access to some super-computer with the capacity to process the consequences of every life event down to the spark of every neuron in the brains of millions of people. It would still not be enough. We would still be faced with an enormous problem of the actual data collection. How does one see into several million people’s heads – all at once, down to the precision of the last conceivably relevant neuronal event? Somewhere along the line, I suspect we would encounter some form of Heisenberg’s uncertainty principle. If so, it may even be that we simply cannot gather enough data – no matter how technically proficient we are.

The difference between the freezing water proposition and the theft proposition is not one of kind, but one of complexity. Both really are just physics problems – dependent on the behavior of physical things that are subject of physical constraints.1 One proposition is within our capacity to assert with something like certainty, while the other simply is not. It should not be surprising that the universe abounds with process we can never fully understand. Nothing we actually know about the universe demands that all the factors relevant to our theft proposition be wholly and simultaneously comprehensible to us. That’s a good deal to ask of three or so pounds of grey matter that did not evolve to solve that sort of problem.

Our understanding of most things (and of almost all things involving human behavior) is, to put it mildly, incomplete. When faced with propositions that are fundamentally beyond our capacity to test, we generally attempt to reduce them to the simplest approximations we can personally tolerate. We content ourselves with probabilities or explanatory constructs.2 How might we attempt to justify our belief in the theft proposition? Typically, we would look for instances in which doubling the length of jail sentences was tried. Armed with such examples, we would create an argument based on probability. However, even if declines in theft were frequently found to follow a doubling in the length of jail sentences, we would still have nothing close to certainty. We would not know whether or not other factors may have accounted for the declines. We would not know whether or not the results were peculiar to the particular cultures in which they occurred. Thus, our “understanding” would rest on the implicit assumption that the correlations we chose to identify trump all other correlations, known or unknown. In other words, we are predisposed to assume that the evidence we have represents the dominant agents of causation, and any evidence that we don’t have is unimportant. This is not a weakness, but an inherent characteristic of the human mind. If we didn’t make such assumptions, we couldn’t make inductions and our brains would not be of much practical use.

Of course, some beliefs aren’t justified with any obvious logic – not even merely inductive logic. If we believe that the theft proposition is true because it plainly makes intuitive sense, we are justifying the belief with a narrative. A narrative is essentially a coherent framework constructed from one’s own prior assumptions and imagination. It depends on the assumption that ideas that are easy to believe are probably true. We can, of course, believe all sorts of silly and erroneous things and still be coherent, because coherence (in the non-philosophical world) only demands we avoid intolerably obvious affronts to logic. Narrative arguments need little evidence; they only need an air of plausibility.3 When faced with problems that are beyond our full comprehension, we actually have no choice but to content ourselves with either oversimplifications or narrative conjectures. Since both of these alternatives are generally presented in the form of truths (typically as declarative statements) they carry more persuasive weight than they can rationally bear.4

We know little about politics, economics, sociology, etc – in the same sense that we know the rudiments of the physical sciences. While history might seem to be an exception to this uncertainty, it too crosses the threshold into conjecture when it attempts to deal in causes rather that the enumeration of mere facts. We can say with reasonable certainty that the battle of Gettysburg took place in July of 1863. When we attempt to explain why it took place, however, we are only making educated guesses. Every historical event, in time, ceases to be a collision of physical causes and becomes an author’s narrative. History tends to become a chronicle of Kings, Presidents and Generals because explaining their behavior is always much easier than explaining the behaviors of the vast, undocumented masses of humanity, let alone the clockwork of the entire physical world.

Given that we can only understand most subjects obliquely and incompletely, it shouldn’t be at all surprising that few of us have exactly the same beliefs. Each of us has a different set of incomplete and problematic data from which to wrest his or her tenuous conclusions. Watch any economics talk show on television and you will see any number of “experts” with any number of substantially different views. All that they make clear, collectively, is the chronically tentative state of their field. The same is true for politicians, psychologists, or anyone else who deals in the murky territory of human interaction.

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1 I am a reductionist in a limited sense. I believe the universe is essentially a machine – a collection of interrelated physical processes. I do not believe, however, that physics (as a discipline) can solve many of the practical problems human beings actually deal with. I have no reason to suppose that Stephen Hawking’s impressive abilities would make him either a good administrator or an effective general, for example.

2 Yes, strictly speaking the freezing water proposition can be considered probabilistic as well. We infer it is a law of nature because of its monotonous regularity. I do not argue that claims like the freezing water proposition are subject to some fundamentally different category of test, but simply that they can be tested. Broadly psychological or sociological problems can rarely even be precisely stated, let alone adequately tested.

3 One need look no further than 9/11 or UFO conspiracy theories for striking examples. While lacking in evidence, these theories mimic the structure of truth. They depict events as causal chains.

4 I do not claim any special dispensation for my own arguments, least of all my political ones.