Many people have goals that will not be achieved during their lifetime. Some may have the goal of eliminating the use of all illegal drugs. Some may want to eliminate all murders, or even all crimes. Some want to eliminate poverty, war, or even eliminate all human suffering.
In my opinion, that would be putting the cart ahead of the horse. Trying to eliminate any of these may well be impossible, or at the very least incredibly unlikely.
The only realistic approach would be instead to focus on reducing the occurrence of these bad things. Manage them, rather then pretending you can eliminate them.
With that out of the way, lets get to the heart of the matter, are unrealistic goals unethical?
I'll suggest that an unrealistic goal doesn't start as unethical. However a goal becomes suspect if all attempted efforts to achieve a goal result in greater harm then those towards a more realistic goal.
If efforts to eliminate something have failed, if the bad thing has instead persisted, perhaps flourished, then we must question whether it was an ethical goal in the first place. We must instead focus on reducing, rather then eliminating the bad thing. We must also strive to reduce the bad effects of the bad thing.
It is for this reason that illegalizing a bad thing is often unethical. This tactic is effective for some bad things, but ineffective for others. Some things will not be prevented by making them illegal. It won't even serve sufficiently as a deterrent. In these cases it merely creates a counter culture, putting large groups of people outside the law.
In these cases we must realize that the unrealistic goal of eliminating the bad thing is unethical. Instead we must focus on the more ethical goal of regulating the bad thing, and honestly educating people on why it is bad. Perhaps we can effectively illegalize certain aspects of the bad thing (restricting it's use to certain areas, times, etc.)
The important thing is realizing that how ethical a goal is may change as it becomes more unrealistic.
Perhaps some goals do not become less ethical as they become less realistic. The golden rule etc.. I myself won't claim all goals that are unrealistic are unethical. I stand by the idea that some goals, with negative consequences, are unethical due to the fact they are unrealistic. An impossible ends can't justify any means.
-Brother Rail Gun of The Short Path
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