One of the major causes of the 2007 global economic crisis was the weak perception of the real risk of it actually occuring. This was due to a "financial optimism" that created the illusion of a sustainable and never-ending prosperity. "Liquid markets' boost growth, but they also prepare the ground for crisis. From where does this short-circuit arise? According to Keynes, it arises from a tacit decision about the role of time in the formation of expectations. This decision shifts the "precariousness of the basis of knowledge" from the facts that happen in time to the precariousness of a "convention", that is the convention that leads financial calculations and expectations. This tacit move from to a real precariousness to an ideological-theoretical one deserves to be made explicit and to be investigated.