A monetary economy, we shall find, is essentially one in which changing views about the future are capable of influencing the quantity of employment … - John Maynard Keynes

Economics is a science of thinking in terms of models joined to the art of choosing models which are relevant to the contemporary world. It is compelled to be this, because, unlike the typical natural science, the material to which it is applied is, in too many respects, not homogeneous through time. The object of a model is to segregate the semi-permanent or relatively constant factors from those which are transitory or fluctuating so as to develop a logical way of thinking about the latter, and of understanding the time sequences to which they give rise in particular cases.
Good economists are scarce because the gift for using "vigilant observation" to choose good models, although it does not require a highly specialised intellectual technique, appears to be a very rare one.

When the facts change, I change my mind - what do you do, sir?

If .. spontaneous optimism falters, leaving us to depend on nothing but a mathematical expectation, enterprise will fade and die.