Evidence of absence

Not to be confused with Absence of evidence.

Evidence of absence is evidence of any kind that suggests something is missing or that it does not exist. A simple example of evidence of absence: A baker never fails to put finished pies on her windowsill, so if there is no pie on the windowsill, then no finished pies exist. This can be formulated as modus tollens in propositional logic: P implies Q, but Q is false, therefore P is false.

Evidence of this kind is not to be confused with mere ignorance, and the traditional axiom warns that "absence of evidence is not evidence of absence", although this is only the case if there is no reason to believe that such evidence would already have been found if it existed. In this regard Irving Copi writes:

In some circumstances it can be safely assumed that if a certain event had occurred, evidence of it could be discovered by qualified investigators. In such circumstances it is perfectly reasonable to take the absence of proof of its occurrence as positive proof of its non-occurrence.

Overview

See also: Distinguishing an "absence of evidence" from "evidence of absence"

The difference between evidence that something is absent (e.g. an observation that suggests there were no dragons here today) and a simple absence of evidence (e.g. no careful research has been done) can be nuanced. Indeed, scientists will often debate whether an experiment's result should be considered evidence of absence, or if it remains absence of evidence. The debate is whether the experiment would have detected the phenomenon of interest if it was there.

The confusion is worsened since arguments from ignorance and incredulity are often (wrongly) advanced in debates as proper "evidence of absence". A case in point: arguing "There is no evidence that this mysterious remedy does not work, therefore it works." Basically, this arguments from ignorance relies on a lack of research to somehow draw conclusions. While this is a powerful method of debate to switch the burden of proof, appealing to ignorance is a fallacy. Carl Sagan criticized such "impatience with ambiguity" using cosmologist Martin Rees' maxim, "Absence of evidence is not evidence of absence."

An exhaustive inspection of the attic for vermin can provide evidence of absence, but any sign of mice will always suffice to the contrary.

In carefully designed scientific experiments, even null results can be evidence of absence. For instance, a hypothesis may be falsified if a vital predicted observation is not found empirically. (At this point, the underlying hypothesis may be rejected or revised and sometimes, additional ad hoc explanations may even be warranted.) Whether the scientific community will accept a null result as evidence of absence depends on many factors, including the detection power of the applied methods, and the confidence of the inference.

According to Pyrrhonian skeptic Sextus Empiricus, who first questioned the validity of inductive reasoning, a universal rule cannot be established from an incomplete set of particular instances:

When they propose to establish the universal from the particulars by means of induction, they will effect this by a review of either all or some of the particulars. But if they review some, the induction will be insecure, since some of the particulars omitted in the induction may contravene the universal; while if they are to review all, they will be toiling at the impossible, since the particulars are infinite and indefinite.

In other words, starting with particular details (e.g. we have seen 4 white swans) one might then make particular or general inferences (e.g. the next swan we see will be white; all swans everywhere are white). Inferences use induction, and must be probabilistic to be realistic, however (e.g. the next swan will probably be white) because induction can never be certain. For instance, there are black swans. That is the problem of induction: unlike deductive proofs, inductions from evidence are never guaranteed to be true.

"You can't prove a negative"

Skeptic James Randi uses the phrase "you can't prove a negative". Philosopher Steven Hales points out that typically one can logically be as confident with the negation of an affirmation.

Hales says that if one's standards of certainty leads them to say "there is never 'proof' of non-existence", then they must also say that "there is never 'proof' of existence either". Hales argues that there are many cases where we may be able to prove something does not exist with as much certainty as proving something does exist.

Existence of God

The existence of one or more deities and relationship between science and religion continues to be debated.

A deity's existence can mean different things to different people. Some claims about the existence of gods or about their actions can be falsified, such as the claim that a god Helios pulls the sun across the sky. Some related claims can be empirically tested: There is evidence of absence for the power of faith healing (which the American Cancer Society also calls potentially dangerous if it replaces proper medical care).

In the media

Gin Rummy, an ex-soldier character in the show The Boondocks, attempts to explain to another character what it means to say "the absence of evidence is not evidence of absence". As Rummy puts it "Simply because you don't have evidence that something does exist does not mean you have evidence that something doesn't exist." When that rephrasing causes confusion, Rummy angrily discusses more confusing but related ideas. Rummy says: "Well, what I'm saying is that there are known knowns and that there are known unknowns. But there are also unknown unknowns; things we don't know that we don't know." This character was parodying a famous line by U.S. Defense Secretary Donald Rumsfeld's media conferences as he was defending the U.S. military attacks upon Iraq in 2002-2003 in a vain search for WMDs.

See also

• Argument from ignorance
• Argument from silence
• Contraposition
• Contraposition (traditional logic)
• Pitfalls
• Transposition (logic)