Evidence of absence

An empty field. If our hypothesis was that dragons would be present, we do not merely lack evidence, but instead possess evidence of absence. (Not really. Actual dragons are invisible.)

Evidence of absence is evidence of any kind that suggests (via certain types of inference or deduction) the non-existence or non-presence of something. A simple example of evidence of absence: checking one's pocket for spare change and finding nothing but being confident that one would have found it if it were there. This is a an example of Modus tollens, a type of logical argument.

In this regard Irving Marmer 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." - Introduction to Logic, Copi, 1953, Page 95

Of course, in practice, it can be difficult to agree whether a particular experiment was a sufficiently "qualified investigation".

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 are no dragons) 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 (e.g. the experiment could have missed what it was looking for).

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" in cosmologist Martin Rees' maxim, "Absence of evidence is not evidence of absence".

To be confident that we have evidence of absence of mice in the attic, we need a confident method of detection

Of course, 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 will be rejected or revised, or even rarely add ad hoc explanations. Whether the scientific community will accept a null result as evidence of absence depends on many things, including the detection power of the applied methods, and the confidence of the inference.

Philosophic arguments that depend on evidence of absence are commonly referred to in peer-reviewed literature as "noseeum arguments." The argument form is specifically inductive in that evidence is accumulated; as one collects a larger dataset the argument grows stronger. Some noseeum arguments are very strong, such as checking the fridge for milk and determining that there is none, since it is relatively easy to systematically remove every item from the fridge, verify that it is not milk, and visually inspect the empty space left over. At the other extreme are noseeum arguments about the existence, or lack thereof, of alien lifeforms. Since the universe is enormous relative to our known area, a noseeum argument stating that there are no alien lifeforms would be very weak.

You can't prove a negative

Notable skeptic James Randi sometimes argued that "you can't prove a negative". He uses the example of looking for Santa Claus as an unprovable negative. According to Philosopher Steven Hales, no logician actually believes this, and the rule may be an oversimplification. As explained above, one can be just as confident about a negative as a positive.

If someone's standards of certainty lead them to believe that there is no 'proof' of nonexistence, then there is no 'proof' of existence for them either.

Hales points out that the second law of thought is a sort of negative; "it is not the case that a thing can be X and not X at the same time". What's more, any argument can be expressed in its negative form. For example, to the extent that you can prove that you are real (a positive argument), you can also prove that you are not imaginary. Whether an argument is phrased as a positive or negative can be arbitrary.

Hales acknowledges that people may mean "you can't prove that something doesn't exist". This claim may be clearer, but remains basically false. First, the claim now concerns issues of "proof" and certainty in general. Hales describes how there is widespread agreement that, as far as humans can be concerned, it seems that any inference is fallible. Neither a negative claim, nor a positive claim, is ever absolutely certain (i.e. absolutely "proving" facts about reality seems impossible). For instance, someone who believes that they have seen Santa Claus may have been hallucinating, or otherwise deceived.

Philosophers acknowledge what is called this Problem of induction, the idea that you never know when new information will prove you were wrong after all. Since inference in general is never certain, it may be more appropriate to say "you can't prove anything, strictly speaking (a negative OR a positive)". Hales also worries that people often appeal to the problem of induction in order to continue believing in less justified beliefs (i.e. pseudoscience).

Gods

Whether humans possess evidence of a God's absence, or simply suffer from an absence of evidence, continues to be hotly debated. Research into the Relationship between religion and science has also yielded mixed opinions.

Philosopher Antony Flew warns that some claims cannot be falsified because they are ultimately contentless. Flew asserts that a first test of content is that any truth claim should imply a situation in which this truth would be disproved. He suggests we ask "What would have to occur or to have occurred to constitute for you a disproof of the love of, or of the existence of, God?" If the proponent cannot provide a negative version of their truth claim, it may have been contentless. Flew also describes how deep questioning, backed up by empirical testing, can reveal a claim to at least have much less content than was originally thought (e.g. it becomes a much less detailed claim) - what he calls "death by a thousand qualifications".

God's existence can mean many things to many different people. Still, some claims (which are actually contentful) about Gods can be falsified, to the extent that they attempt to describe any facts about physical reality. For instance, the creationist claim that God created humankind in their current form has been falsified by evolution. There is therefore, in this case, at least evidence of absence of a certain kind of God: one who created humans in their current form. Many (but not all) other claims related to God can be similarly tested (e.g. the practice of praying to cause someone to heal has also been recognized, among scientists, as having been disproved).

Other examples

Some other instances of evidence of a thing's absence include: A biopsy may show the absence of malignant cells (evidence of absence of a tumour), and the result of Michelson–Morley's famous experiment is interpreted as "strong evidence" that the luminiferous aether does not exist. Another case in point, close inspection of an attic may reveal no sign of vermin infestation and therefore evidence of vermin absence in the attic. Note however that a critic might argue, if one did not open every available box, that one still possesses an absence of evidence.

More information

See also

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

Further reading

• Van Inwagen, Peter (2006), "LECTURE 8 THE HIDDENNESS OF GOD", The Problem of Evil: The Gifford Lectures given at the University of St Andrews in 2003, Oxford University Press, p. 173, ISBN 0199245606 “If the present argument appeals to any general epistemological principle, it is this rather obvious one: If a proposition is such that, if it were true, we should have evidence for its truth, and if we are aware that it has this property, and if we have no evidence for its truth, the fact that we have no evidence for its truth, is (conclusive) evidence for its falsity.”
• Johnstone, Albert A. (1991), Rationalized Epistemology: Taking Solipsism Seriously, SUNY Press, p. 39, ISBN 079140787X “The reason why the everyday view P is not known (at least in as much as it contradicts the skeptical thesis) is not merely that the evidence for P is insufficient to warrant a knowledge claim; it is that the evidence for P is inexistent. In the absence of any such evidence, there is no reason at all to think that P is true. To claim that P is true is consequently to make a purely gratuitous claim.”