deletions | additions       diff --git a/Social_costs_socially_optimal_level__.md b/Social_costs_socially_optimal_level__.md  index 757ebc9..360f2ee 100644  --- a/Social_costs_socially_optimal_level__.md  +++ b/Social_costs_socially_optimal_level__.md 
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private costs + externality costs    "the idea is to get back to 4 units from 5 units"  you're still getting pollution - you're not getting rid of pollution = but what you say is "it's okay to pollute as long as the benefits are outweighing the costs"  eg. do the costs of noisy students outweigh the displeasure of the silence-loving librarian? What does this mean for astrophysics?