Your keys are missing. Suppose I said that they were probably stolen by “key gnomes” in your house.
You: I’ve never seen any key gnomes in my house.
Me: Well . . . that’s because key gnomes are invisible.
You: Why are they invisible?
Me: Uh, so they can steal more keys!
You: What do they eat?
Me: Keys.
You: How do they digest metal?
Me: They have a special gland, found only in key gnomes, that secretes powerful acid in their stomach.
You: [face palm]

This illustrates what I would call an “ad hoc” defense of a hypothesis.

An Explanation

Here’s how Jim Pryor, eminent philosopher from NYU, describes the concept of ad hoc:

“You call something ad hoc when it’s introduced for a particular purpose, instead of for some general, antecedently motivated reason. . . . [A]d hoc hypotheses . . . are hypotheses (or stipulations or amendments) adopted purely for the purpose of saving a theory from difficulty or refutation, without any independent motivation or rationale. They will usually strike the reader as artificial or “cheating.”

For instance, suppose you analyze “bird” as “any creature that can fly.” I then cite mosquitos as a counter-example. They can fly, but they aren’t birds. Now, you might fix up your analysis as follows:

• A bird is any creature that can fly, and which is not a mosquito.

This would be an ad hoc response to my counter-example. Alternatively, you might fix up your analysis as follows:

• A bird is any creature that can fly, and which has a backbone.

This would be an independently motivated, and more appropriate, response to my counter-example. (Of course, someone may discover counter-examples even to this revised analysis.)”

Theism v. Atheism

In my recent YouTube live conversation with Tom Jump, this issue came up (at about 59:33).We were discussing whether a theistic model (roughly of the Judeo-Christian sort) or non-theistic (or a quasi-theistic, but impersonal) model would better explain certain features of our universe. I suggested that a personal being would be a better explanation for the existence of conscious minds, and for the temporal finitude of the universe. Tom replied that a non-personal force could explain all the same features. Consequently, I pushed back by saying that his move here–suggesting that the universe could make free choices just as well as a divine being–was ad hoc.

Recall the bird example given by NYU philosopher Jim Pryor above. Improving the classification of birds by including “has a backbone” seems quite natural and fitting. Backbones are a natural and unifying feature of birds, unlike color or size. Similarly, there is something quite fitting and appropriate to include the ability to make free choices in our description of a divine being. By contrast, to suggest that a quantum particle, or the universe itself, has the ability to make free choices seems odd, or as J.L. Mackie would call it, “queer.” It appears to be “cheating,” as Pryor says. Amending one’s hypothesis in such a bizarre and inconsistent way is ad hoc.

(For additional discussion of ad hoc reasoning, listen Dr. Kenny Boyce on my podcast, starting about 21:25.)

Why This Matters

I think this matters for two reasons. First, it matters because it makes a difference in the dialectic between Christian theism and atheism or other non-personal varieties of ultimate reality. I think “perfect being” theism is superior to its rivals as an explanation for the universe we find ourselves in. The inability of non-personal ultimate reality, without ad hoc amendments, to adequately account for the existence of consciousness or apparent design is one reason for this.

Second, understanding ad hoc reasoning also helps us to avoid making bad arguments in other areas of life as well. We fabricate ad hoc alterations just to save face when our goal is winning, rather than truth. Or when we feel terrified of the embarrassment of being wrong. I like this humorous example from logicallyfallacious.com :

Frieda: I just know that Raymond is just waiting to ask me out.
Edna: He has been seeing Rose for three months now.
Frieda: He is just seeing her to make me jealous.
Edna: They’re engaged.
Frieda: Well, that’s just his way of making sure I know about it.

Here’s another one from the Internet Encyclopedia of Philosophy:

Yolanda: If you take four of these tablets of vitamin C every day, you’ll never get a cold.
Juanita: I tried that last year for several months, and still got a cold.
Yolanda: Did you take the tablets every day?
Juanita: Yes.
Yolanda: Well, I’ll bet you bought some bad tablets.

In all these examples, people strain imagination to save their claim. The better thing to do is admit that you have a problem, and resolve to do a little homework on it. There’s no need to instantly give up your view, because it may very well be correct. But if after investigating the problem, you can’t find a non-ad hoc solution, it may be time to give it up.