Renewed interest in international institutions makes clear the need for a better theory of institutional possibilities. Friedrich Hayek held that institutions are either designed, though badly, or emerge spontaneously, and providently, as an unintended consequence of agents' self-interested choices. Hayek's historical sketch misses a third set of possibilities reflecting the claim that agents make institutions in keeping with nature's design or, as we say today, make them on some occasions to suit large social purposes. The English School treats institutions as spontaneous developments. Liberal scholars in the US start with the rationalist position that agents design institutions as they see fit, but end up closer to the view that institutions constitute a purposive whole.