An ad hominem circumstantial is a fallacy in which one attempts to attack a claim by asserting that the person making the claim is making it simply out of self interest. In some cases, this fallacy involves substituting an attack on a person's circumstances (such as the person's religion, political affiliation, ethnic background, etc.). The fallacy has the following forms:
Person A makes argument X.
Person B asserts that A makes argument X because it is in A's interest.
Therefore claim X is false.
Person A makes claim X.
Person B makes an attack on A's circumstances.
Therefore X is false.
An ad hominem circumstantial is a fallacy because a person's interests and circumstances have no bearing on the truth or falsity of the claim being made. While a person's interests will provide them with motives to support certain claims, the claims stand or fall on their own. It is also the case that a person's circumstances (religion, political affiliation, etc.) do not affect the truth or falsity of the claim. This is made quite clear by the following example: "Bill claims that 1+1=2. But he is a Republican, so his claim is false."
There are times when it is prudent to suspicious of a person's claims, such as when it is evident that the claims are being biased by the person's interests. For example, if a tobacco company representative claims that tobacco does not cause cancer, it would be prudent to not simply accept the claim. This is because the person has a motivation to make the claim, whether it 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 obviously does not make her claim false.
- "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."