Logical ideas often fail because logic demands universally applicable laws but humans, unlike atoms, are not consistent enough in their behaviour for such laws to hold very broadly. For example, to the despair of utilitarians, we are not remotely consistent in whom we choose to help or cooperate with. Imagine that you get into financial trouble and ask a rich friend for a loan of £5,000, who patiently explains that you are a much less needy and deserving case for support than a village in Africa to which he plans to donate the same amount. Your friend is behaving perfectly rationally.
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