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I don’t think they could actually stop such a technology from being deployed on earth in such a scenario. Nor do I think they would really object, it seems doubtful to me that they would all want to go to mars, and for those that don’t, trying to prevent it’s use is counter to their interests.
But the reasoning given doesnt apply exclusively to horses. Suppose we follow the same chain that gets us “all horses are the same color”, but replace “horses” with “colors”, we would end up with the statement that all colors are the same color. Thus, this is not a counterexample, because black and brown are the same color.