**PHYSICAL LAWS AND STATISTICAL LAWS**
In the aggregate of natural phenomena that form the object of scientific inquiry, two groups may be distinguished. The first consists of all those phenomena whose succession occurs with unvarying regularity, so that—starting from the observation of the conditions in which, at a given instant, the phenomenon occurs—one can always express its past and future evolution by means of mathematical equations: thus one arrives at the formulation of a “physical law,” which links, in a well-defined and constant relation, the quantities involved in the phenomenon. Laws of this type have been obtained in the study of classical mechanics, electromagnetism, etc.
There exists, however, a vast complex of phenomena, belonging to the second of the categories mentioned above, for which the formulation of physical laws proves impossible. Their interpretation is achieved through empirical rules, the so-called “statistical laws,” which are arrived at by renouncing the investigation of the development of individual phenomena and limiting oneself to the determination of average values, deduced from the observation of a large number of phenomena of the same type. Unlike what occurs with physical laws, statistical laws do not allow one to deduce with absolute certainty the evolution of the phenomenon in all its details; they merely permit the prediction of its average course with a very high degree of probability.
Statistical laws govern thermal processes, the disintegration of radioactive substances, and the phenomena that form the object of quantum mechanics. The reasons why physical laws cannot be applied to this complex of phenomena are not the same in all cases. Precisely in thermodynamic systems, the intervention of statistical laws is necessitated by the enormous number of their constituents (molecules), which makes the investigation of the evolution of the molecules one by one practically unfeasible; nonetheless, no one can *a priori* exclude the theoretical possibility of knowing the physical laws that govern these individual phenomena. In the other phenomena listed above, however, the application of statistical laws is precluded by the principle of indeterminacy, which asserts the impossibility of formulating physical laws from which one could deduce the behavior of individual phenomena.
It should finally be noted that statistical laws are not—contrary to what might at first appear—less rigorous in practice than physical laws. To realize this, it suffices to recall how physics, like all experimental sciences, never deals with absolute quantities, since every measurement, no matter how precise, is always subject to error.