How do people make decisions? How do we make progress in science? And how do courts rule? Underlying these processes is a simple yet far-reaching logic that few people know. Understanding it gives you greater visibility into many seemingly unrelated areas.

Consider the following quote by philosopher Karl Popper about science:

A serious empirical test always consists in the attempt to find a refutation, a counter example. In the search for a counter example, we have to use our background knowledge [...].

— Conjectures and Refutations, p. 325 (emphasis added)

I argue that, in addition to the sciences, background knowledge plays a crucial role in all rational endeavors. (Surely Popper knew this – I'm just pointing it out.) Its role extends even to the judicial realm and others, as I will explain shortly.

We begin, as is customary in Popperian epistemology, with problems. A problem, with physicist David Deutsch, is a conflict between ideas.¹ In its simplest form, a problem can always be defined as a conflict between an idea and its negation. Given an idea A and its negation !A (read: 'not A'), if we are to make a decision, we need to take sides. We do so by searching for a refutation of either A or !A in our background knowledge; whichever of the two survives may then be incorporated into our background knowledge later on. As Popper writes (ibid, p. 323):

While discussing a problem we always accept (if only temporarily) all kinds of things as unproblematic: they constitute for the time being, and for the discussion of this particular problem, what I call our background knowledge. Few parts of this background knowledge will appear to us in all contexts as absolutely unproblematic, and any particular part of it may be challenged at any time, especially if we suspect that its uncritical acceptance may be responsible for some of our difficulties. But almost all of the vast amount of background knowledge which we constantly use in any informal discussion will, for practical reasons, necessarily remain unquestioned; and the misguided attempt to question it all—that is to say, to start from scratch—can easily lead to the breakdown of a critical debate.

In other words, our background knowledge is vast; much of it is not related to our problem situation. Considering it uncontroversial, or 'unproblematic' as Popper calls it, is a practical concern: we simply cannot question it all at once. That would be a form of 'starting over' – a revolution – which should be avoided. In short, for something to be uncontroversial means you have no reason to doubt it for the moment. And because it is uncontroversial, we can use it to refute one of the conflicting ideas: they are both considered controversial for the moment, so the uncontroversial idea can serve as a 'tiebreaker'. It's a tie because there's a symmetry, as philosopher Elliot Temple calls it, between the conflicting ideas. Taking sides is an act of breaking that symmetry.

An idea could be about anything. For example, consider the problem of having lost your keys. To find them, you guess they're on the kitchen counter – this is your idea A. Now you need to find some way of determining (fallibly) whether A is true or !A is true. You do this by finding another idea in your background knowledge, or by creating a new idea, that conflicts with either. For example, you could check the kitchen counter. In this scenario, your sense data is uncontroversial: if you see your keys on the counter, you can refute !A in favor of A, and you (fallibly) conclude that your guess was correct. (That sense data is often (but not always!) uncontroversial has fooled many into thinking that knowledge is derived from the senses – which is the mistaken doctrine of empiricism.)

Here's a slightly more involved example: somebody gives an outrageous quote. Should you believe the quote? You could ask for a screenshot of the source. If the screenshot accurately captures the quote, you may believe it. But if somebody then tells you that screenshots are notoriously unreliable because they're easy to fake, you may realize you were mistaken to consider screenshots uncontroversial, in which case you're back to where you started.

Keys and screenshots may sound like mundane examples, but they follow the same logic as any decision-making process. And because they involve empirical data, they're also scientific.

What about other areas? I'm no lawyer, but I believe background knowledge is of utmost importance in court proceedings. Consider a defamation lawsuit in which the plaintiff claims that the defendant made libelous statements about the plaintiff in an email. How can the court determine (as always only fallibly) whether the statements were made in the fashion the plaintiff reported? It could ask the plaintiff to produce the emails. But that wouldn't be uncontroversial – he's one of the two conflicting parties, ie he represents one of the two conflicting ideas. Following the logic I laid out above, the court needs a third, independent idea or representative thereof to decide.

For example, the court could subpoena the defendant's email provider. That email provider is an independent third party with no incentive to lie and thus represents part of the court's background knowledge. If, on the other hand, the plaintiff's lawyer finds that the defendant bribed his email provider to produce false records, then, just like in the case of easily faked screenshots, the court is back to square one. Whether it believes the plaintiff's lawyer depends on whether he can refer to shared background knowledge showing that the email records have indeed been manipulated. (I suspect all persuasion involves the successful use of shared background knowledge.) If he makes a convincing case, what the court originally considered unproblematic is, in fact, highly controversial.

The logic is simple, but finding relevant background knowledge and devising adequate tests to take sides between conflicting ideas can be difficult. I suspect this difficulty is the reason why courts issue a threat of overwhelming punishment for perjury. The punishment is intended to scare a potentially dishonest party into submission so that their statements can be considered uncontroversial, thus facilitating legal decision-making. However, it should be noted that such threats indicate a lack of knowledge and thus are a sign of incompetence.² I'm no expert, but I understand they're often found not to work anyway: people still lie in court, maybe because they think they can get away with it.

Likewise, when you sign a contract, it may say that you must pay x amount of dollars in damages if you break it. The signature is then considered uncontroversial if the dollar amount would hurt sufficiently. In addition, those threatening punishment have to make good on their threats, or else their threats are not considered uncontroversial; that is, when someone decides between, say, breaking a contract and not breaking it, the enforcer wants him to find it uncontroversial that he would follow through on his threat.

Since background knowledge is crucial in adjudicating conflicts between ideas and thereby choosing between theories, we can see now that there can be an epistemological need for punishment in certain situations where the requisite background knowledge is lacking. It would be pessimistic, however, to think that punishment is the only way to come by such background knowledge, and other, non-tyrannical ways should be preferred when they are expedient.

When a court decides whether a given law is unconstitutional, the logic is again the same: idea A says the law is constitutional, whereas !A says it is not. The constitution, itself unproblematic in this constellation, represents background knowledge. The court can then review the constitution, ie consult its background knowledge, to make a decision between A and !A and to rule accordingly.

In all these cases, the question is not 'how can I use background knowledge to be certain I made the right decision?' Instead, its use should have a Popperian focus on error correction: 'How can I use background knowledge to eliminate errors? How can I get closer to the truth?' While the veracity of an idea is always a binary matter, its 'uncontroversialness' is a matter of degree. We can always feel more or less certain that some idea is uncontroversial, only to be disappointed (or excited, depending on your attitude) once we realize we were mistaken after all. Therefore, background knowledge cannot serve as a criterion of truth – all we can hope for is that we use it correctly on occasion.

Note that, when we refer to existing background knowledge to decide between ideas, we need not be creative. But sometimes we lack the requisite background knowledge, and we have to come up with a new idea. When a cop tells you it's your word (A) against the other guy's (!A), he's saying that there is currently no background knowledge to decide which one of you is right – so you better come up with a way to prove your case. Understanding how a mind creates new ideas is the study of artificial general intelligence. As Deutsch points out,³ the creation of new ideas and alternatives is a crucial part of decision-making, which shouldn't be viewed as a process that is restricted to weighing a fixed set of pre-existing ideas.

It's a great success when a true idea is adopted and becomes background knowledge. That the earth is round was once considered crazy, now it's considered self-evidently true. We can only hope that other important ideas, eg that children should be free, will soon enjoy the same fate.

Thanks to Travis Smith for discussing the ideas in this article before publication.