My eye recently caught on t.v., in passing, an old film clip of someone trying to fly like a bird. This would-be aviator had wing-like things strapped to his arms. Of course, no amount of flapping would get this human being to take flight like the birds he sought to imitate. One important reason, of course, is that we humans – as smart and clever as we are – can observe only a tiny fraction of the details of what enables birds to fly. We can observe a handful of large details (“bird flaps limbs that extend kinda, sorta from bird’s shoulder”; “bird’s flapping limbs are made of lightweight things that we call ‘feathers’,” and so on). But the amount of detail that we don’t – can’t – observe with the naked eye (even with a naked eye as careful as that of Leonardo) is overwhelming. The bird’s musculature; cardiovascular system; weight and position and minuscule maneuverings of its tail – these and countless other relevant details aren’t observed.
The human being observes a bird in flight and then analogizes his – the human’s – limbs and muscle movements to what he supposes, from his observations, are those of the bird. But the human is too easily misled into thinking that he can easily-enough mimic the bird’s body and movements and, thereby, achieve flight.
Obviously, no human can do so. Such attempts as there have been by man to strap on wings, flap, and fly invariably failed – sometimes catastrophically for the pretend Icaruses.
Attempts to centrally plan economies are very much like attempts to fly by dressing like and flapping like a bird: utterly futile because the most that can be observed of any successful economy are a handful of large details (“assembly lines,” “retail outlets”…..). The vast majority (99.99999999999…9 percent) of the details that must work reasonably well  aren’t observed by the would-be central planner. What Hayek called  “knowledge of the particular circumstances of time and place” – knowledge of details spread today across the globe and across billions of different human minds – is not incidental to the successful operation of a modern economy. Utilizing that knowledge – vast, deep, changing, incredibly fine-grained detailed knowledge – is the very key to a successful market economy.
Central planning is as futile as trying to strap on wings and fly like a bird – and potentially as calamitous.
Of course, few people today advocate full-scale central planning of economies. But smaller-scale interventions suffer the same problems as do attempts at central planning: inevitably inadequate knowledge of how to intervene. Asserting, for example, that the key to economic recovery is to “increase aggregate demand” is a fiction borne from observing a true, but only large and inadequate, fact about successful economies: most producers, at any given time, are able to sell most of what they plan to sell. But to leap from this observation to the conclusion that “therefore, government can stimulate economic recovery by increasing aggregate demand” is akin to a human being costumed-up as a bird and leaping off of a mountaintop, flapping away, hoping, hoping, hoping to fly.