Ethics derived from utopian reasoning also tends to be ethics fit for the rulers of said utopia, or at least rulers striving to make their society into said utopia. Most of us not being rulers, it tends to lead to impractical conclusions with respect to what should be done - ie conclusions that ignore what I/you/we can do from our actual positions of agency and (small) power.

It is similar to the tendency of identifying with the rich (more obvious in the US, but clearly existing on this side of the pond to).

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