Is psychohistory possible?
Is a science of history, in fiction also called psychohistory, possible? I don’t think a simple “yes” or “no” can answer this question, unless one believes in unlimited free will. There is a saying that “You can do anything!” If true, psychohistory is not possible, and even the laws of physics are a mere illusion. If so, world hunger and all the negative things in the world exist because no one has chosen to immediately end such suffering. Gravity, the speed of light limitation, conservation of energy-mass—all these constraints can be ignored by the open-minded individual possessing unlimited free will.
If the laws of physics do impinge upon free will to any degree at all, then one could go further and ask “To what extent is psychohistory possible?”
The laws of thermodynamics can apply to just a few particles, but the fewer the particles, then the less predictable the system becomes. In Asimov’s Foundation, character Hari Seldon (the founder of psychohistory in that series) was able to predict the course of major human history centuries into the future within an error of just a few percentage points. Given the amount of noise in the progression of societies both on Earth, and fictionally even in the galactic society in the Foundation series, it is difficult to conceive such accuracy. Aside from social reasons, there are pure physical reasons. The occurrence of the exact timing of supernovas in a galactic society centuries in advance seems inconceivable, and and yet supernovas can have a tremendous impact upon life in galactic regions.
Also, the connections between human individuals and between their social structures also moves us away from the “gas” particle distribution analogy that apparently inspired Asimov. Gas particles are substantially independent of each other, and thus their collective behavior is easier to predict. (That may seem paradoxical, but that is how statistical mechanics works.) In contrast, for large and really large societies, the “gas” particle distribution analogy has more validity. Yet unlike gasses, human societies often have centralized control, adding an additional constraint, and reducing the number of “particles” for some purposes.
The conservation of energy-mass (modern version of the First Law of Thermodynamics) would constrain the actions of a human society. Populations must be fed or else they die. Large mountains can rarely be moved, so they must be tunneled or flown over in order to transverse them. Further, the Second Law of Thermodynamics (that the entropy of a system of a system cannot decrease) further limits the efficiency of social-economic processes, such as how much useful work can be obtained from grain or an engine. For example, in petroleum and metals mining, decreases in efficiency have often bankrupted mines and their surrounding communities despite the presence of significant remaining resources in such wells and mines. Entire dynasties and regimes have fallen victim to falling Second Law efficiency.
There are laws of psychology that may also serve as constraints on behavior and efficiency. Often in history, people rather die than compromise their values. Although such psychology may have altruistic emotional appeal, it cannot be reasonably denied that such psychology results in social inflexibility and hence constraints.
In summary, a science of history, or psychohistory, is possible to the extent that physical and psychological constraints exit upon a society exist and their effects can be accurately modeled.