Peter Suber, "One-Sidedness Fallacy"

The One-Sidedness Fallacy Peter Suber, Philosophy Department, Earlham College
This is one of the most common and most misleading fallacies. It really ought to have a name. Some writers call it special pleading, but most writers use that term for a slightly different fallacy. Some call it confirmation bias, which is an accurate but little-used term. I like "one-sidedness fallacy" because we are accustomed to calling arguments "one-sided" if they suffer from the limitations we'll describe here.
The fallacy consists of giving reasons for your thesis without considering reasons against it, or giving reasons against an opposing view without considering reasons for it.
It's easy to say something for virtually any thesis, or to say something against it. So to hear something for or against a thesis doesn't take us very far. To be in a good position to decide the truth of a thesis, we'd like to hear (1) the best that can be said (2) on each side. We'll worry about "the best" elsewhere. This hand-out is about reaching two-sidedness.
Note that there may be far more than two sides to a complex issue. So the true alternative to one-sidedness is many-sidedness. But I will refer to the alternative as "two-sidedness" for convenience.

The one-sidedness fallacy does not make an argument invalid. It may not even make the argument unsound. The fallacy consists in persuading readers, and perhaps ourselves, that we have said enough to tilt the scale of evidence and therefore enough to justify a judgment. If we have been one-sided, though, then we haven't yet said enough to justify a judgment. The arguments on the other side may be stronger than our own. We won't know until we examine them.
So the one-sidedness fallacy doesn't mean that your premises are false or irrelevant, only that they are incomplete. You may have appealed only to relevant considerations, but you haven't yet appealed to all relevant considerations.
Some logicians say that an argument is cogent if it is valid and sound and takes all relevant considerations into account. On this usage, one-sidedness does not undermine validity or soundness, but cogency.

To become two-sided, you must first make the arguments against your own thesis explicit. Write them out as carefully as you write out the positive arguments for your thesis. But if that were all, your final case would be indecisive or inconsistent. You must take the counter-arguments into account. Demonstrate their weaknesses, admit their strengths, and revise your own argument accordingly. In practice this takes many forms. It might mean answering the counter-arguments and showing their inadequacy. It might mean retracting part of your thesis or one of your arguments for it. It might mean qualifying an unqualified or oversimplified thesis. It might mean acknowledging an exception. It might mean making a concession. It will almost always mean making a simple argument complex.
The remedy for one-sidedness is either experience or imagination, or both, and a willingness to use them. Either you've encountered arguments on the other side in your life or your reading, or you must imagine them. You know that you can imagine things that you don't believe —ghosts, gremlins, Godzilla. Do that here. Just as an exercise, imagine that you deny the thesis that you believe. What arguments can be mustered for that denial? Don't let the playfulness of this exercise mislead you about its importance. It opens your mind.
Your imagination will give you some of the arguments against your thesis, but perhaps not the best ones. These tend to come from people who are living the circumstances you can't imagine. So expect to strengthen the two-sidedness of your arguments with a lifetime of sensitive listening and observation.
Our courts avoid one-sidedness by giving trained professionals on each side of a case a serious interest in making the strongest arguments they can for their side. Each lawyer might make a one-sided argument but the judge and jury get a two-sided body of evidence and reasoning.
Even here, however, the lawyer is not only more likely to understand the facts, but to win, if she makes a two-sided argument. An argument for one side that disregarded the strong arguments on the other side leaves the jury to wonder whether you can answer them.
This leads to an important point. You might think that one-sidedness is actually desirable when your goal is winning rather than discovering a complex and nuanced truth. If this is true, then it's true of every fallacy. If winning is persuading a decision-maker, then any kind of manipulation or deception that actually works is desirable. But in fact, while winning may sometimes be served by one-sidedness, it is usually better served by two-sidedness. If your argument (say) in court is one-sided, then you are likely to be surprised by a strong counter-argument for which you are unprepared. The lesson is to cultivate two-sidedness in your thinking about any issue. Beware of any job that requires you to truncate your own understanding.

This file is an electronic hand-out for the course, Real-World Reasoning.