The next argument scheme we will look at is what's known as the argument from ignorance. An argument from ignorance (or argumentum ad ignorantium if you want to be fancy) is one that asserts that something is (most likely) true because there is no good evidence showing that it is false. It can also be used the other way to argue that a claim is (most likely) false because there's not good evidence to show that it's true.
Lets look at a couple of (valid) examples:
There's no good evidence to show that the ancient Egyptians had digital computeres. (This evaluation comes from professional archeologists), therefore, they likely didn't have digital computers.
There's no good evidence to suppose the earth will get destroyed by an asteroid tomorrow. (This evaluation comes from professional astronomers), so we should assume it won't and plan a picnic for tomorrow.
There's no good geological evidence that there was a world-wide flood event. (This evaluation comes from professional geologists), therefore we should assume that one never happened.
Formalizing the Argument Scheme
As you may have guessed, we can formalize the structure of the argument from ignorance:
P1: There's no (good) evidence to disprove (or prove*) the claim.
P2: There has been a reasonable search for the relevant evidence by whomever is qualified to do so.
C: Therefore, we should accept the claim as more probable than not/true.
C*: Therefore, we should reject the claim as improbable/false.
Good and Bad Use of Argument from Ignorance
The argument from ignorance is philosophically interesting because sometimes the same structure can be used to support the opposing position. The classic example is the debate over the existence of God. Lets look at how both sides can employ the argument from ignorance to try to support their postion.
P1: You can't find any evidence that proves that God or gods don't exist.
P2: We've made a good attempt to find disconfirming evidence, but can't find any!
C: Therefore, it's reasonable to suppose that God or gods do exist.
Vs God Arg
P1: You can't show any evidence that God or gods do exist.
P1*: Any evidence you present can also be explained through the natural laws.
P2: We've made a good attempt at looking for evidence of God's/gods' existence but can't find any! (I even looked under my bed!)
C: Therefore, it's reasonable to suppose that God/gods don't exist.
This particular case brings out some important issues we studied earlier in the course such as bias and burden of proof. Not surprisingly, theists will find the first argument convincing while atheists will be convinced by the latter. This of course brings up questions of burden of proof. When we make a claim for something's existence, is it up to the person making the claim to provide proof? Or does the burden of proof fall on the critic to give disconfirming evidence? In certain questions, your biases will pre-determine your answer.
While in the above issue, there is arguably reasonable disagreement on both sides, there are other domains where the argument from ignorance fails as a good argument. As you might guess, this will have to do with the acceptability of P1 (i.e., there is/is no evidence) and P2 (i.e., a reasonable search has been made). Most criticism of arguments from ignorance will focus on P2--that the search wasn't as extensive as the arguer thinks. Generally, we let P1 stand because it is usually an authors opinion to the best of their own knowledge. Recall from the chapter on determining what is reasonable, we typically let personal testimony stand.
We can illustrate a poor example of an argument from ignorance with an example. Claim: There's no evidence to show Obama is American, therefore he isn't American.
Lets dress the argument to evaluate it:
P1: I've encountered no good evidence to show that Obama is an American citizen.
P2: Numerous agencies and individual trained in the search and identification of state documents have been unable to locate any relevant documents.
C: Obama isn't American (and is a Communist Muslim).
Regarding P1, maybe the arguer hasn't encountered any evidence so we'll leave it alone. P2 however has problems. There have been reasonable searches for evidence, and that evidence was found. Perhaps, the arguer was unaware or didn't truly exert him/herself enough. The argument fails because P2 is not acceptable (i.e., false).
We can also typically find the argument from ignorance used in arguments against new (or relatively new) technologies in regards to safety or efficacy. For example:
We should ban GMOs because we don't know what long-term health effects are.
P1: I've found no evidence that shows that GMOs are safe for human consumption.
P2: Those qualified to do studies and evaluate evidence have found no compelling evidence to show that GMOs are safe for human consumption.
C: Therefore, we should assume GMOs are unsafe and ban them until we can determine they are safe.
If we were to criticize this argument we'd consider P2. In fact, there have been quite a few long term studies done by those qualified to assess safety. At this point we will have a debate over quality of evidence. Some on the anti-GMO side dispute the quality of the evidence (i.e., it was funded by company x, and therefore it is questionable). In a full analysis we'd consider this question in depth, but for our purposes here, we might legitimately challenge the claim that there is no available evidence purporting to demonstrate safety.
As an aside, notice that we can also use the argument from ignorance for the opposite conclusion: There's no compelling evidence to show that GMOs are unsafe for human consumption in the long-term, therefore, we should continue to make them available/ should not regulate them.
The "team" that wins this battle of arguments from ignorance will have much to do with our evaluation of P2: That there legitimately is or isn't quality evidence one way or the other.
Final Notes on Arguments from Ignorance
We can look at arguments from ignorance as probabilistic arguments. That is, given that there is little or no evidence for something, what is the likelihood that it still might exist? This is especially true for claims that something does exist based on an absence of evidence for its non-existence. However, as Carl Sagan famously said, "absence of evidence is not evidence of absence." In other words, just because we can't find evidence for something, doesn't mean that the thing or phenomena doesn't exist.
On the flip side, this line of argument can also be used to support improbable claims. Consider such an argument for the existence of unicorns or small teapots that circle the Sun: There's no evidence that unicorns don't exist or small tea pots don't circle the Sun, therefore we should assume they exist.
At this point we should return to the notion of probability: Given no positive evidence for these claims, what is the probability that they are true (versus the probability that they aren't)? It seems that, given an absence of evidence, the probability of there being unicorns is lower than the probability that they do not exist. Same goes for the teapot.
Typically, in such cases we say that the burden of proof falls on the person making the existential claim. That is, if you want to claim that something exists, the burden is upon you to provide evidence for it, otherwise, the reasonable position is the "null hypothesis." The null hypothesis just means that we assume no entity or phenomena exists unless there is positive evidence for its existence. In other words, if I want to assert that unicorns exist, using the argument from ignorance won't do. It's not enough for me to make the claim based on an absence of evidence. This is because, we'd expect some evidence to have turned up by now if there were unicorns (i.e., P2 of the implied argument would be weak).
This brings us to another Carl Sagan quote (paraphrasing Hume): "Extraordinary claims require extraordinary evidence." Or as Hume originally said: "A wise man proportions his beliefs to the evidence." Claiming that unicorns exist is an extraordinary claim and so we should demand evidence in proportion to the "extraordinariness" of the claim. This is why an ad ignorantium argument fails here; it doesn't offer any positive evidence for an extraordinary claim, only absence of evidence. We'll discuss this principle of proportionality more in the coming section. For now, just keep it in mind when evaluating existential arguments from ignorance.