Economics’ aspirations to be like physics would succeed if only economic constructs meant the same thing and worked the same way across time (but they never do).

Inspired by Marshall’s rejection of the distinction between natural and market prices in the preface to his Principles of Economics:

And as there is no sharp line of division between conduct which is normal, and that which has to be provisionally neglected as abnormal, so there is none between normal values and “current” or “market” or “occasional” values. … For the element of Time, which is the centre of the chief difficulty of almost every economic problem, is itself absolutely continuous : Nature knows no absolute partition of time into long periods and short; but the two shade into one another by imperceptible gradations, and what is a short period for one problem, is a long period for another.

Another tidbit: The problem of finding a unit of value that was stable across time haunted Ricardo until the end of his life.