Ad Hominem Tu Quoque

Also Known as: “You Too Fallacy”

Description:

This fallacy, which is a type of Ad Hominem, has two versions. The first occurs when it is concluded that a person’s claim must be false because it is inconsistent with something else the person claimed. The second occurs when it is inferred that a person’s claim must be false because it is inconsistent with their actions. The first version has the following form:

 

Version 1

Premise 1: Person A made claim X.

Premise 2: Person A also made claim Y.

Premise 3: It is asserted that claim X is inconsistent with claim Y.

Conclusion: Therefore, X is false.

 

This fallacy might seem to have some logical appeal, especially since it (mis)uses the notion of logical consistency. Consistency and inconsistency are logical relationships between two (or more) claims. If two claims are consistent, then they can both be true (but both could be false) at the same time.  For example, the claim that my water bottle has liquid in it and the claim that it has water in it are consistent: they can both be true. If two claims are inconsistent, then while both could be false, they cannot both be true at the same time. As an example, the claim that my water bottle contains only water is inconsistent with the claim that it contains vodka. While both cannot be true, both could be false. The bottle could, for example, be empty. If you know that two claims are inconsistent, you know that at least one of them must be false. However, and this is the fundamental error of this fallacy, the inconsistency does not tell you which claim is false (and both could be false. As such, when it is concluded that a specific claim must be false, this is an error in logic. It would be good reasoning to conclude that at least one of the claims must be false if they are inconsistent but that is another pattern of reasoning that would look like this:

 

Good Reasoning About Inconsistency

Premise 1: Claim A and Claim B are inconsistent.

Conclusion: A or B (or both) is false.

 

There are also cases in which it is wrongly claimed that two claims are inconsistent, but this mistake (or intentional deceit) is distinct from this fallacy.

While it is less common, this fallacy can also be committed using contradictory claims rather than inconsistent claims. Two claims are contradictions if they both cannot be true at the same time, but both cannot be false at the same time. As an example, the claim that my water bottle is empty contradicts the claim that my water bottle contains something. One of these must be true and one must be false. As with inconsistent claims, knowing that two claims are contradictory does not automatically inform you which one is false. All you know is that one is true, and one is false.

 

Version 2

Premise 1: Person A made claim X.

Premise 2: Person A does Y.

Premise 3: It is asserted that claim X is inconsistent with doing Y.

Conclusion: Therefore, X is false.

 

This reasoning is fallacious because failing to act in accord with one’s professed belief does not show that the belief is false. A person who professes beliefs and then acts in ways inconsistent with them could be subject to moral judgement and they might be a hypocrite. But this would not disprove their claims.

This fallacy can have considerable psychological force. This is because people are usually expected to act in accordance with their professed beliefs. If someone does not do so, this might incline us to think they are lying. That is, because they seem to be lying about what they believe, we might wrongly infer that their claim is a lie. In some cases, a person can be lying about what they believe. For example, a person might say they believe that free speech is a right for everyone, but they do not and favor laws that silence people they dislike. While such deceit can be condemned on moral grounds, it does not prove that the person’s claim is not true. Free speech, in this example, might be a right for everyone and the person is a hypocrite.

In other cases, a person might really believe what they claim, but fail to follow their professed belief for any number of reasons. For example, someone might believe (correctly) that running is good for their health, but they might lack the time or inclination to run. Their failure to run would not prove that running is not good for you, just that the person is not acting on their belief.

 

Defense: The defense against the first version of this fallacy is to keep in mind that while inconsistent claims cannot both be true, this inconstancy does not (by itself) show which claim is false. And both might be false. In the case of the second version, the defense is to remember that an inconsistency between a person’s actions and their claim does not show that their claim must be false.

 

Example #1:

Bill: “Smoking is very unhealthy and leads to all sorts of problems. So, take my advice and never start.”

Jill: “Well, I certainly don’t want to get cancer.”

Bill: “I’m going to get a smoke. Want to join me, Dave?”

Jill: “Well, I guess smoking can’t be that bad. After all, Bill smokes.”

 

Example #2:

Jill: “I think the gun control bill shouldn’t be supported because it won’t be effective and will waste money.”

Bill: “Well, just last month you supported the bill and you said it would be effective. So, I guess you’re wrong now.”

 

Example #3:

Peter: “Based on the arguments I have presented; it is evident that it is morally wrong to use animals for food or clothing.”

Bill: “But you are wearing a leather jacket and you have a roast beef sandwich in your hand! How can you say that using animals for food and clothing is wrong!”