Let not anyone pacify his conscience by the delusion that he can do no harm if he takes no part, and forms no opinion. Bad men need nothing more to compass their ends, than that good men should look on and do nothing.
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Gender-inclusive version:

Let not anyone pacify one’s conscience by the delusion that one can do no harm if one takes no part, and forms no opinion. Bad people need nothing more to compass their ends, than that good people should look on and do nothing.