Evidence of absence



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 was 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
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. It is to such impatient, inappropriate conclusions that Carl Sagan referred when he said "Absence of evidence is not evidence of absence".

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.

Examples
Some 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.

Whether humans possess evidence of 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 yielded mixed opinions. Of course, certain definitions of Gods are far more often believed to have been falsified in the scientific community than others. For example, many agree that we possess evidence of absence of Poseidon's existence - who once filled gaps in scientific knowledge about the weather and oceans.