Critics of anti-speciesist arguments, like presumably the Foundation for Biomedical Research in my previous post, often fall back onto simplified hypothetical moral situations in order, typically, to elicit an inconsistency in the opponent's belief sets or between their beliefs and actions. These arguments typically take the form of asking "your dog or your child?" On the basis of the inconsistency, there are a number of possible consequences that they might wish to suggest or draw, including:
a) Opponent's anti-speciesist view is false.
b) Opponent's anti-speciesist view is weakened.
c) Opponent is an unreliable judge of the moral issue due to the incoherence of their belief set.
d) Opponent is an unreliable judge of the moral issue due to hypocrisy.
As we've commented before, these Subjunctive Tu Quoque arguments are often fallacious, though sometimes they have some probative significance (e.g., by dialectically shifting the burden of argument). But, there is another case of argument that looks like the Subjunctive Tu Quoque, but operates differently.
P1: Opponent S asserts p, either generally or in situation A, on moral ground U.
P2: But, opponent S would assert ~p, in situation B.
C3: Opponent S should not hold p on moral ground U either generally or in situation A.
Here's an example from philosopher Carl Cohen.
"Tom Regan enjoys outdoor activities, and we can well imagine that on some cross-country hike a child of his may be bitten by one of the Eastern diamondback rattlesnakes abundant in those North Carolina woods, or a cottonmouth, or copperhead perhaps, or during a winter holiday in Martinique his wife may be struck by the fer-de-lance, a snake whose bite is often fatal if not swiftly treated with an antivenin. Happily, there is treatment readily available for such excruciatingly painful bites, an antivenin that is waiting for the Regan family or any family in need of it, at any good hospital in North Carolina or the Caribbean. But would Tom Regan's child be allowed to receive it? Here is the problem. The needed treatment for the bites of the family of pit vipers is Antivenin (crotalidae) Polyvalent-serum globulin obtained from the blood of healthy horses that have been injected with snake venoms to cause of the development, in their blood, of the needed antibodies. Those horses have been used without their consent, with some pain to them. But, if the antivenin is not administered quickly, children bitten by rattlesnakes (or other pit vipers) will suffer terribly, may lose an arm or leg, or even die." (Carl Cohen and Tom Regan, The Animal Rights Debate, Rowman and Littlefield, 2001. p 242.)
Here Carl Cohen is following out what he takes to be the consequence of Tom Regan's animal right's position. I don't think that he is actually arguing against Regan's view here, instead he is setting out the practical stakes of Regan's position before examining "with a very skeptical eye the philosophical arguments by which it is claimed that 'animal rights' are established" (p.243).
But, one might formulate an argument as follows:
P1: Tom Regan asserts that animals have rights not to be used for human purposes generally.
P2: But Tom Regan would/should assert that humans have the right to use animals (horses) in situation B.
C1: Tom Regan should not hold that animals have rights not to be used for human purposes generally.
or, C2: Tom Regan's judgment in P1 is unreliable.
This argument might have a similar structure as the standard reconstruction of the Socratic Elenkhos articulated by Gregory Vlastos, an instance of the Argument from Inconsistency. Though in the Elenkhos, we would add some additional premises to which the interlocutor agrees that entail C1, or for Socrates (on Vlastos' interpretation) the stronger claim that the original belief (animals have rights not to be used for human purposes generally) is false (Vlastos, Gregory "The Socratic Elenchos" Journal of Philosophy 79 (11), 1982, 711-714).
But, there is an important difference between arguments of this sort and the reconstructed implicit argument of FBR's billboard. In the case of Cohen's hypothetical, the hypothetical is an instance of the principle in question. In the case of FBR's billboard, the hypothetical is not. To put it simply:
Cohen: Using horses to produce anti-venom is a counter-example to the principle that animals have rights not to be used.
FBR: Saving a little girl rather than a rat is not an instance of the general category of using animals in research.
So, FBR cannot, I think, defend the implicit argument by modelling it on an implicit Elenctic argument. Conclusions about the use of animals in research is a non-sequitur from the assumed answer to the billboard's question.