A fallacy of ignorance is an argument based on a lack of evidence to support or discredit an idea. It can be used to argue that something is true because there is no evidence to the contrary, or that something is false because there is no evidence to support it. However, not all statements of this kind are fallacies, as some require a burden of proof to be met. It is important to distinguish between fallacies and reasonable inferences based on available information.
A lapse of ignorance is an argument intended to support or discredit an idea based on the lack of evidence to the contrary. Someone might, for example, argue that since there is no evidence that fact X is false, fact X must therefore be true. A similar argument could be made in the other logical direction, and one may argue that since there is no evidence to show that fact Y is true, fact Y must therefore be false. However, there is not always a fallacy of ignorance when making this type or argument, as there are some cases where the burden of proof must be met for an argument to be valid.
Also called an argument of ignorance or a plea of ignorance, a fallacy of ignorance can take many forms, but is always based on a lack of evidence to support an argument or idea. One of the more common arguments made using this fallacy is that if there is no evidence that something isn’t true, then it must be true. A fallacy of ignorance is often used to argue a position such as “since you can’t prove aliens on other worlds don’t exist, then they do.”
This same kind of fallacy of ignorance can be used on the other side of an argument, however, to argue that something isn’t true simply because it hasn’t been proven. Someone might argue that “since you can’t prove aliens exist in other words, then they mustn’t exist”. In many arguments, like this one, the same fallacy can be used to support both sides of an issue. This kind of fallacy of ignorance also functions as a special form of fallacy of one or the other, as it works under the assumption that only two conditions can be accurate: either that it’s true or it’s not true.
It is important for someone studying and considering logical fallacies to keep in mind that not all statements of this kind are a fallacy of ignorance. In US law, for example, the burden of proof rests with the prosecution and it is natural to assume that if they cannot provide sufficient evidence of guilt, then the defendant is not guilty. Even outlandish or illogical claims typically need to be well supported or proven, or else people are likely to dismiss them by the same method of reasoning.
Many people operate under a “closed world assumption,” where information that points to what is known can be used to discredit or ignore what is not known. Movie viewing times, for example, indicate when movies will be played, and it can logically be assumed that times not mentioned are times when they will not be played. This is not a fallacy of ignorance, but merely a means by which information can reasonably be inferred under certain circumstances.
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