A fallacy is a kind of error in reasoning. Fallacies should not be persuasive, but they often are. Fallacies may be created unintentionally, or they may be created intentionally in order to deceive other people. The vast majority of the commonly identified fallacies involve arguments, although some involve explanations, or definitions, or other products of reasoning. Sometimes the term "fallacy" is used even more broadly to indicate any false belief or cause of a false belief.

An informal fallacy is fallacious because of both its form and its content. The formal fallacies are fallacious only because of their logical form. For example, the Slippery Slope Fallacy has this form: Step 1 often leads to step 2. Step 2 often leads to step 3. Step 3 often leads to ... until we reach an obviously unacceptable step, so step 1 is not acceptable. That form occurs in both good arguments and fallacious arguments. The quality of an argument of this form depends crucially on the probabilities that each step does lead to the next. Notice that the probabilities involve the argument's content, not merely its form.

Below some detail is provided by one form of fallacy – argument from ignorance. (For information about the other hundreds of types of fallacies, see the links at the bottom of the page.)


The argument from ignorance (or argumentum ad ignorantiam) is a logical fallacy that claims the truth of a premise is based on the fact that it has not been proven false, or that a premise is false because it has not been proven true. This is often phrased as "absence of evidence is not evidence of absence".

Logical Form:

X is true because you cannot prove that X is false.
X is false because you cannot prove that X is true.

Example #1:

Although we have proven that the moon is not made of spare ribs, we have not proven that its core cannot be filled with them; therefore, the moon’s core is filled with spare ribs.

Explanation: There is an infinity of things we cannot prove -- the moon being filled with spare ribs is one of them. Now you might expect that any “reasonable” person would know that the moon can’t be filled with spare ribs, but you would be expecting too much. People make wild claims, and get away with them, simply on the fact that the converse cannot otherwise be proven.

Example #2:

To this very day (at the time of this writing), science has been unable to create life from non-life; therefore, life must be a result of divine intervention.

Explanation: Ignoring the false dilemma, the fact that we have not found a way to create life from non-life is not evidence that there is no way to create life from non-life, nor is it evidence that we will someday be able to; it is just evidence that we do not know how to do it. Confusing ignorance with impossibility (or possibility) is fallacious.

Other forms

Another form that this fallacy can take is the form that of an argument from incredulity (also known as argument from personal belief or argument from personal conviction) which is that one's personal incredulity or credulity towards a premise is a logical reason for acceptance or rejection. This incredulity can stem from ignorance (defined as a lack of knowledge and experience) or from wilful ignorance (defined as a flat out refusal to gain the knowledge). The concept of irreducible complexity is based entirely around this idea of personal incredulity. One person (Michael Behe) cannot see how something evolved naturally, therefore it can't possibly evolve naturally.

Use in arguments

Almost all the claims from the anti-science movement revolve around some form of personal incredulity or argument from ignorance.

Proponents of the anti-science movement will usually pick some aspect of a currently accepted scientific theory and argue that it must be wrong because they do not believe it explains some aspect of the natural world. Common examples of this are such claims as "you can't prove global warming is caused by humans," "I don't see how evolution could increase the complexity of an organism," "material properties of the brain cannot presently explicitly explain consciousness so it must be caused by non-materialist processes," or "I don't know how this alternative medicine works, but it does."

God of the gaps

God of the gaps (or a divine fallacy) is logical fallacy that occurs when Goddidit (or a variant) is invoked to explain some natural phenomena that science cannot (at the time of the argument). "God of the gaps" is a bad argument not only on logical grounds, but on empirical grounds: there is a long history of "gaps" being filled and the gap for God thus getting smaller and smaller, suggesting "we don't know yet" as an alternative that works better in practice; naturalistic explanations for still-mysterious phenomena are always possible, especially in the future where more information may be uncovered.

The god of the gaps is a didit fallacy and an ad hoc fallacy, as well as an argument from incredulity or an argument from ignorance, and is thus an informal fallacy.

Endless gaps

When each gap is filled, the believer is forced to jump to the next gap. This game can continue ad nauseam, since human knowledge will never be able to explain everything (by definition of infinity, and by principles such as Gödel's incompleteness theorems).

However, the argument is an instance of the logical fallacy of argument from ignorance.

The ultimate "gap" that likely cannot be bridged is "well, god started everything", because even if something like the m-theory explaining how our universe could have "big banged" in the first place was proven to be true someone could always ask, "yes, but what created the membranes?".

Gap multiplication

Creationists generally declare that, rather than filling a gap, a new piece of information simply generates two gaps, one on either side of the newly-established fact -- meaning that additional information is understood to diminish the observational base of a theory. As such, increases in knowledge would paradoxically increase human ignorance.

Endless knowledge

The God of the Gaps argument indicates enormous conceit because, by implication, a believer indicates that he (or she) has understanding of all there is, except those things God did, and therefore declares that a miracle is necessary to make him (or her) fail to understand. It needs hardly to be said that this belief system has little do to with observation, and much to do with blind belief in the unknown.


"Science doesn't know everything" (also "Science can't explain X") is an argument that asserts that, because of science's lack of knowledge about something, something else must be true. The implication is that, because science does not have an answer (or a sufficiently good answer) already, any claim can take its place, even though it has no supporting evidence. The argument is closely linked to the Science was wrong before and God of the gaps arguments.

The argument is overused by woomongers and theists alike, and is used to disparage the application of scientific methods to problems under discussion or analysis.

The argument is an informal fallacy and an argument from ignorance.

Common forms

Generally the argument follows one of the two following forms:

P1: If science can't explain X, then X is true.
P2: Science can't explain X.
C1: X is true.


P1: If science can't explain X, then Y is true.
P2: Science can't explain X.
C1: Y is true.

This is an informal fallacy, because the conclusion does follow if the premises are true. Just because science can't explain something, doesn't mean it's true.

Occasionally the arguer will forget/imply the 1st premise, for a (formally fallacious) form of:

P1: Science doesn't know everything.
C1: X is true.

Common usage has, rather than "X is true", an equivocation of "is true" with "is likely true", "could be true" or "you can't prove it isn't true".

For a fuller list of fallacy types see the links below: