One of the fundamental axioms of economics is that we know what we like: we have preferences, they're consistent across time and they can be revealed by careful experimentation. This, of course, is utter nonsense. Yet it's not just a guiding principle of economics but is also a rough and ready rule we live our lives by. We make decisions and then we justify them, after the event, because to do otherwise would make a mockery of our choices.
But because we do actually make decisions we must, in a sense, really know what we like, even if we're habitually inconsistent. Unfortunately, outside of our own specialised areas of expertise we don't know how to absolutely value things and all too often we assign a value based on entirely superfluous data intermingled with a bit of relative valuation, reckoning that a Porsche 911 Carrera is worth more than a child's teddy bear. We exhibit coherence but in an arbitrary fashion - a behavior known, rather unoriginally, as coherent arbitrariness.