Carl has just pointed out that researchers who want to experiment on animals have to complete detailed pain protocols, whereas humans undergoing operations do not. From this comparison, Carl concludes that if lawmakers were as concerned about human beings as they are about animals, humans would have pain protocols too. By the counterfactual, this means Carl concludes that, since there are no pain protocols for humans, lawmakers must not be as concerned about human beings as they are about animals.

Debbie responds to Carl's argument by pointing out that humans can communicate directly with doctors about the pain they will experience from an operation, and decide from there whether they will go through with the process. From this premise, she concludes that pain protocols are unnecessary for humans.

It may seem like Debbie's argument isn't a direct response to Carl's. Carl's conclusion is about the motives of lawmakers, whereas Debbie doesn't even mention lawmakers. However, Debbie's argument undermines Carl's conclusion in a different way. Notice that Carl's argument relies on an analogy: a comparison between animals and humans, with the unstated assumption that practices used for animals should also be used for humans, at least if people are equally concerned about both. Debbie points out that Carl's comparison doesn't work because there is an important difference between humans and animals: humans, unlike animals, can give informed consent to pain they will experience. From this, Debbie concludes that pain protocols are unnecessary for human beings.

Notice how this conclusion affects Carl's argument: if pain protocols aren't necessary for humans, a lawmaker could be equally or more concerned about human beings than animals and still not enforce pain protocols for humans, since they aren't relevant. In other words, the absence of pain protocols doesn't tell us anything about the concern of lawmakers for humans versus animals, which is a rejection of Carl's conclusion.