Critical Reasoning (CR) | Re: It is the central tenet of the ethical theory of the German Idealist

The answer is E.

We can find the answer using the

Logical

approach, since this question is about figuring out the premises for the argument given.
The argument takes a claim Kant made about humans and extrapolates it to animals. Therefore, since our goal is to weaken the claim, we will look for a fundamental difference between humans and animals that makes this extrapolation illegitimate. This is exactly what (E) gives us: Kant's conclusion about treating humans as ends rests upon the assumption that humans
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