 Angela Kallhoff has offered an alternative, albeit similar, definition of a public good (Kallhoff 2011: Ch. 2). According to her, a public good is one that satisfies the “basic availability condition” and the “open access condition”. Basic availability is her analogue of non-rivalry. A good satisfies this condition whenever each person benefitting from it has access to the same amount and the same types of benefits. To state the condition in these terms makes explicit that there may exist a level of consumption at which sameness of amount and types of benefit are no longer guaranteed. A single ship’s enjoying the benefits of a lighthouse does not affect another ship’s doing the same but once the sea is congested this may no longer be the case. A lighthouse is a public good because there exists a level at which each person has access to the same amount and the same types of benefits. Open access is Kallhoff’s analogue of non-excludability. It states that 