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This two pillars (non-excludability and non-rivalry) approach is very 
common, 
but in teaching I lean harder on one pillar, non-rivalry. This is because 
public goods are generally thought of as a species of market failure.  Not 
including cases of non-rivalry where we have excludability therefore seems 
to 
imply that the market provision is hunky-dory here. But if I manage to find 
a 
technology which allows excluding non-paying ships from the light of my  
lighthouse, so as to make it a profitable venture for me to produce it, it  
is still inefficient to charge above the marginal cost of zero. (Not to  
mention the deadweight nature of the costs of inventing such a  
technology!)  It is especially important in this so-called "age of  
information" to make sure people understand the inefficiency of charging 
for the provision of non-rivalrous goods. 
 
Kevin Quinn 
 
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