A fallacy of ignorance is an argument meant to support or discredit an idea based on the fact that there is a lack of evidence to the contrary. Someone may, for example, argue that because 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 someone can argue that since there is no evidence proving that fact Y is true, fact Y must therefore be false. A fallacy of ignorance does not always occur when this type or argument is made, however, since there are some instances in which the burden of proof must be met for an argument to be sound.
Also called an argument from ignorance or appeal to ignorance, a fallacy of ignorance can take several different forms but always relies on a lack of evidence to support an argument or idea. One of the most common arguments made using this fallacy is that if there is no proof that something is not true, then it must be true. A fallacy of ignorance is often used to argue a position such as “since you cannot prove that aliens on other worlds do not exist, then they do exist.”
This same type of fallacy of ignorance can be used on the other side of an argument, however, to support that something is not true simply because it has not been proven. Someone might argue that “since you cannot prove that aliens on other words do exist, then they must not exist.” In many arguments, such as this one, the same fallacy can be used to support both sides of an issue. This type of fallacy of ignorance also functions as a special form of either-or fallacy, since it works under the supposition that only two conditions can be accurate — either that it is or is 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 is on the prosecution and there is a natural assumption that if it is unable to provide sufficient proof of guilt, then the defendant is not guilty. Outlandish or illogical claims must also typically be well-supported or proven, otherwise people are likely to dismiss them under this same method of reasoning.
Many people operate under a “closed world assumption,” in which information that indicates what is known can be used to discredit or ignore what is not known. Show times for movies, for example, indicate when movies are going to play, and a logical assumption can be made that times not mentioned are when they will not play. This is not a fallacy of ignorance, but simply a means by which information can be reasonably inferred in certain circumstances.