A Circumstantial ad Hominem is a fallacy in an attack on a person’s circumstances (such as the person’s religion, political affiliation, ethnic background, etc.) is substituted for evidence against a claim. There is a more specific version in which a claim is attacked by asserting that the person asserting the claim is doing so only from self-interest. The fallacy has the following forms:
Premise 1: Person A makes claim X.
Premise 2: Person B makes an attack on A’s circumstances.
Conclusion: Therefore, X is false.
Premise 1: Person A makes claim X.
Premise 2: Person B asserts that A makes claim X because it is in A’s self-interest to claim X.
Conclusion: Therefore, claim X is false.
This is a fallacy because a person’s self-interest or circumstances have no bearing on the truth or falsity of the claim being made. While a person’s self-interest gives them a motive to support certain claims, the claims stand or fall on their own merits.
A person’s circumstances (religion, political affiliation, etc.) do not affect the truth or falsity of their claims. To use a silly example: “Bill claims that 1+1 =2. But he is a Republican, so his claim is false.” This is absurd.
There are times when it is prudent to suspicious of a person’s claims when there is evidence that they are biased. For example, if a tobacco company representative claims that tobacco does not cause cancer, it would be prudent to not simply accept the claim on their word. This is because the person has a motivation to make the claim, whether the claim is true or not. However, the mere fact that the person has a motivation to make the claim does not make it false. For example, suppose a parent tells her son that sticking a fork in a light socket would be dangerous. Simply because she has a motivation to say this does not make her claim false.
A person’s self-interest and other biasing factors can affect their credibility, and these are reasonable to consider when making such an assessment. For example, if you learn that a once seemingly credible expert has been receiving money from renewable energy lobbying firm, then this would reduce their credibility. But it would not prove that what they have said about renewable energy is false. This is discussed in the Appeal to Authority.
Defense: The defense against this fallacy is to distinguish between legitimate concerns about a person’s credibility due to biasing factors and mere attacks on their circumstances. Even if a person is biased, it does not follow that their claim is false. This fallacy can be both self-inflicted and used against you by others.
“She asserts that we need more military spending, but that is false, since she is only saying it because she is a Republican.”
“I think that we should reject what Father Jones has to say about the ethical issues of abortion because he is a Catholic priest. After all, Father Jones is required to hold such views.”
“Of course, the Senator from Maine opposes a reduction in naval spending. After all, Bath Ironworks, which produces warships, is in Maine.”
“Bill claims that tax breaks for corporations increases development. Of course, Bill is the CEO of a corporation.”
Kelly: “I’m buying solar panels. While they cost up front, I’ll be getting free electricity. I’ll also help reduce climate change. I mean by some insanely small fraction, but every bit helps.”
Ted: “What gave you the idea?”
Kelly: “Well, my electrical engineering professor was talking about solar in class. She sold me on the idea.”
Ted: “You mean Dr. Lee?”
Ted: “You know that these big solar companies help fund her research. You know they give her all those solar panels and batteries.”
Kelly: “Damn. No solar for me! All that stuff she said must be crap.”