Absence Of Evidence Is Not Evidence

Evidence Collection

Photo Attribution: Main121 (public domain)

Rule It Out First

There’s an interesting sentiment that I hear on occasion when conversing with atheists about God. I’ve been informed that absence of evidence is evidence of absence. I admit that it seems to be convincing, but only to those that have no training in formal logic. After all, if there’s no evidence for God (in the form of tangibility -usually the type requested by atheists) then this absence seems to be a blow to his existence. Right? Not exactly. We’re going to examine the modus tollens argument (Latin; the way that denies by denying) to see if this type of reasoning is valid or not. The argument goes like this:

  • If X is true then Y is true.
  • Y is false.
  • Therefore X is false.

A modus tollens argument is not based on absence of evidence, but rather the presence of contrary evidence that contradicts what we would expect to see if a proposition were true. Example, the reason people can know that I’m not a leprechaun is not because of the absence of evidence that I am a leprechaun, it’s the presence of evidence that I’m not a leprechaun. There’s good evidence that shows I’m a normal full grown human. The atheist philosopher Austin Dacey says: “What if these arguments purporting to establish that God exists are failures? . . . Must we then conclude that God does not exist? No. Lack of supporting reasons or evidence for a proposition does not show that the proposition is false. [1] He is clearly keeping the modus tollens in mind with such a statement.

Here’s another example which shows that absence of evidence is not evidence of absence. Let’s say that I come home from work and find out that my drumset is missing. The absence of evidence that it was stolen (e. g. no broken windows; no signs of forced entry etc) doesn’t automatically mean it was not stolen. It could have been, but it also could have been borrowed by my brother in law or wife etc. The same can be said for a crime scene investigation where there’s a person laying dead in his bed. The absence of evidence that it was murder (verses suicide or natural causes), doesn’t rule out that it wasn’t murder. In reality, it very well could be murder but the evidence may never surface.

___________________________

Endnotes

1. The Case for Humanism, 2003, pg. 162

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About Razor Swift

Rich Christian who is the founder of Razor Swift, seeks to open hearts and minds through the platform of apologetics. It's his desire to approach Biblical, faith, and other issues from a different perspective rather than just preaching to the choir in the Christianese dialect. He maintains that faith and reason mustn’t necessarily be at odds with each other, but can be complementary. May no stone lay unturned.
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12 Responses to Absence Of Evidence Is Not Evidence

  1. hbhatnagar says:

    This is a slogan that cuts both ways, and I don’t think any thinking person would be employing it in a debate. Whichever side of the debate one may be on, it’s the preponderance of evidence that settles the issue not the absence thereof per se. However, this does not mean that all claims/explanations carry equal weight in a situation where evidence is scarce either.

  2. I prefer “extraordinary claims require extraordinary evidence”.

  3. “Well, since the universe clearly had a beginning”

    The universe as we know it seems to have had a beginning with the Big Bang.

    I make no claim to knowledge of what happened to cause the Big Bang, or what, if anything, happened before it.

    “(that one would need extraordinary evidence for -according to your standard, but there is none)”

    Certainly it would require extraordinary evidence. Which is why some scientists are searching for said evidence.

  4. A. Poster says:

    DEAD THREAD REVIVAL!

    To clarify what is likely meant with the phrase absence of evidence is evidence of absence, It’s likely meant in a probalistic sense, rather than a neccesistic sense.

    For example, in the above example you gave of the missing drumset, the fact that there was an absence of evidence of forced entry, etc. Wouldn’t NECCESARILY imply that it wasn’t stolen, but it would PROBALISTICLY imply that it wasn’t. (In this case it would be more likely that a roommate or someone had moved it, say)

    The key thing here is that it’s dealing with probability (with the cardinal example being the bayesian interpretation), rather than deductive neccesistic logic, where things are merely more likely, rather than guaranteed.

    The main thing to keep in mind here though, is that even though absence of evidence does probalisticly imply evidence of absence of evidence, it usually only WEAKLY implies it.

    So when employing this argument against the existence of something, it usually doesn’t have a whole lot of strength to it.

  5. A. Poster says:

    To further elaborate on it usually only being WEAK evidence of absence, take for example.

    A city slicker is taking a short hike thru the woods and is pondering the existence of deer. They reason that if deer existed, then they’d see one. They don’t see one, thus they conclude deer don’t exist.

    In this example, because of the relative sloppiness of their experiment, the fact that they saw no deer only works as weak evidence in favor of their hypothesis. More likely would be the hypothesis that deer do exist, but the noisy cityslicker just scared them all away.

    Now a case where this kind of probabilistic reasoning could be much stronger is in the case of if there was an elephant in my room, I’d see it. I don’t see an elephant, therefore there isn’t one.

    Some people might consider the elephant example a case of valid necessistic Modus Tollens, and in principle it is, but in real life, it becomes (very strong) probalistic, as there is always the possibility (admittedly very slight in this case) that my test failed and I got a false negative, namely that I overlooked the elephant.

    A better real world example of this kind of reasoning that is less certain is that of medical tests to see whether someone has a condition or not.

  6. A. Poster says:

    So to sum up, few things, if any are COMPLETELY certain, but partial certainty is usually achievable.

    Although probalistic arguments against the existence of supernatural entities do have some substance, they tend to be weak overall.

  7. A. Poster says:

    So a person ends up not too far from square one (neutral/undecided) in terms of where the evidence lies.

  8. A. Poster says:

    Okay, one last necropost and then I’ll be done.

    I’d conjecture that the main thing that determines the strength of the evidence and hence of the condfidence required would depend upon the strength of the test(s).

    In the city slicker example, the test was weak, and therefore only counted as weak evidence. While in the elephant example, the test was very strong.

    When tests are applied to the supernatural, things are lot more iffy. For the nonexistence of bigfoot, which if it did exist would be a tangible creature, likely leaving ample tracking sign (prints, scat, dna evidence from hairs, etc.) The fact that this evidence doesn’t exist, it can be considered strong evidence for the nonexistence of said creature.

    However, when applied to the existence of god, which tends to be intangible by definition, any tests which could be done, if any, would be a lot weaker and offer a lot less confidence.

    For anyone wanting to learn more about this, look up bayes theorem (but avoid yudkowsky’s primer, it’s hellishly long and convoluted and the man himself is kinda kooky)

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