There are few fallacies more common than that of suppressed evidence.
The fallacy of suppressed evidence is committed when a person argues for a certain conclusion using premises favorable to one side and ignoring available premises that would favor the other side.
Think of it in terms of the oath witnesses take in court, to tell “the truth, the whole truth, and nothing but the truth.” That last clause-“nothing but the truth”-addresses direct, explicit lying, “deception by commission,” one could say. But don’t underestimate the importance of the middle clause-“the whole truth.” That clause means you are not to leave out relevant information just because it might hurt your side, “deception by omission,” one could say.
Cogent reasoning requires basing one’s conclusions on all available relevant information. You commit a fallacy when you neglect to consider some of that information.
Let’s say you’re trying to get a friend to attend a certain movie with you by pointing out all the factors you think will encourage him to go (it’s a comedy, it got good reviews, it’s directed by someone whose previous movies your friend has liked), but you’re careful not to mention other factors that might discourage him from going (it’s in a foreign language with subtitles, the theater will be very crowded).
In this case you’re committing the suppressed evidence fallacy. Not because anything you told him was untrue, or was irrelevant to his decision of whether to go to the movie, but because of what you didn’t say.
This fallacy is pretty much ubiquitous in public discourse (which is one of the reasons public discourse, and all that depends on it, is in such a sorry state).
Returning to the courtroom, the American judicial system is structured as an “adversarial” one, where-with certain limitations-the two sides not only are allowed to commit the suppressed evidence fallacy, but are required to. Each side is expected to present all and only the evidence that supports the outcome it wants.
(Now, we should step back and note that, as a point of legal or political philosophy, proponents of the American judicial system will make the case that we should not be troubled by the fact that-taken in isolation, taken out of context-the individual parties each commit the suppressed evidence fallacy, because collectively, institutionally, the adversarial system itself avoids the fallacy. That is, rather than obligating each individual in the system to provide all the ingredients necessary for institutional cogent reasoning, the system gets different ingredients from different parties. Whether in the end this point is deemed persuasive, it does render it at least debatable whether the adversarial system can be condemned as fallacious after all.)
But beyond that, the vast majority of the information you receive through public means is from agenda-driven shills whose job it is to advocate for certain points of view or encourage certain behavior. Advertising is the most blatant example of this. There are (minimal) restrictions on out-and-out lying in advertising, but there are certainly no rules against committing the suppressed evidence fallacy. There is no requirement, for instance, that a McDonald’s commercial give you all the available relevant evidence so you can make the most informed decision of whether to dine at their restaurant; it’s understood that they will spin their message in whatever way serves their interests.
A spokesperson for an organization, a politician, a pundit from some ideological “think tank,” even some “journalists,” are not honest folks trying to help us be a more informed citizenry by providing all relevant available evidence. They are engaged in polemics, in advocacy. They are presenting limited information in whatever ways will further their ends.
Not that garden variety lying is all that rare from these parties, but what you especially need to be on the defensive about is not getting the whole picture, of being shown only those aspects of it that favor those who have a big enough soapbox to address the general public in the first place.
With the suppressed evidence fallacy, though-and really this is true of fallacies in general-we aren’t talking just about how people improperly try to persuade other people, but also how people’s own reasoning is flawed.
So, yes, part of being a critical thinker is defending yourself against the nefarious efforts of others to lead you astray, but another part of it is policing your own thought processes.
For instance, it’s also an instance of the suppressed evidence fallacy to look only at factors that put you and your behavior in the best light so that you can avoid ever feeling guilty or damaging your fragile self-esteem. Or to look only at material you’re confident will agree with your present political or religious beliefs.
If anything, because there is such a strong psychological tendency to be more receptive to information that fits with our pre-existing worldview (especially in whatever areas happen to be most emotionally powerful for us), we may need to compensate for this by bending over backwards to seek out and fairly consider information for “the other side.”
One final note: Avoiding the suppressed evidence fallacy (and avoiding fallacies in general) absolutely does not require that one be neutral on all issues, or on all controversial issues, nor that one refrain from coming to conclusions. There is a very unfortunate tendency to equate things like “objectivity” or “lack of bias” with refraining from expressing an opinion on matters of controversy. (Why this tendency exists is an interesting matter for sociological speculation, but that goes beyond the scope of this piece.) This is just plain wrong. Sound reasoning is a matter of how you come to your conclusions, not whether you do. If you take into account all available relevant evidence, and you reason cogently from that evidence to your conclusions, then you are not guilty of bias. Quite the opposite, in fact.