Jeremy Taylor - Intuitions are fair game in philosophy. Think of thought-experiments which elicit various intuitions. E.g. one cannot object to the Gettier cases by saying "We have a theory of knowledge which shows that these cases are cases of knowledge." Do you have the intuition that Pruss's counterexample is permissible?

musiclover wrote:

"...it seems plausible that it would be acceptable to temporarily deprive an organ of its natural function.

Yes, seems so.  I think Feser holds that this is fine, and that the NL prohibits only using a factulty in such a way that actively thwarts its natural telos.  

I think with the sense of smell, an example would be finding the smell of a chemical that causes anosmia very pleasant, and indulging in sniffing the chemical for pleasure in such a way that one destroys one's sense of smell.

As suggested, it's seems that natural law theory hold that is immoral to use a faculty contrary to the faculty's end, not that it's immoral not to use the faculty per se all the time. For example, a end of the legs is locomotion, but sitting is not morally wrong. In the same vein, the end of the reproductive organs is reproduction, but a man doesn't need to spend 24 hours per day in procreation. Turning off a sense of smell by a period seems akin to that.