If those that believe in material determinism (i.e.,natural scientists) are right, planning of an economy is just an illusion. It was all predetermined by the strict laws of nature. If those that believe in free will are right, planning of an economy is impossible. You cannot plan someone else's choice. That's just impossible by definition. In either case, the "planners" are just fooling themselves.
Note that I am
not saying planning is useless. I am saying that planning of other people's
choices is a logical contradiction. Planning of your own actions is not a
contradiction. Thus, like any one of us, the planners can only plan
their own actions, close their eyes and hope for the best. But, your planning
of your own actions is not the same as planning of the economy.
people have free will, then the degree of our "influence" on them is a matter of
their choice. A more accurate description of what people mean (or ought to mean) when they talk about planning within society is--coordination of
individual plans. Every individual plans his/her own actions while
taking into account what he/she believes about the plans of others.