in geology, a method in the natural sciences for the study of the earth’s development. Actualism comes from the proposition that the “present is a key to the understanding of the past.” It is an integral part of the comparative historical method which has found wide application in geology. Actualism, based on the concepts of the interaction of the composition of mountain types, the specific aspects of the environment, and the dynamics of geological processes, permits science to utilize the data of present-day natural phenomena and the effects of their action for the purpose of inferring the nature of the ancient geological conditions in which rocks were formed. The term actualism appeared in the German geological literature during the second half of the 19th century and found widespread application during the 1920’s. Understanding of the concept changed over the years with changes in the approach to the connection of past and present.

The rudiments of actualism appeared at the earliest stages of human knowledge, in ancient myths, biblical passages, and the writings of the ancient naturalists. Actualism was later applied in the analysis of geological history, at first irrationally—in Middle Asia by Avicenna and al-Biruni, 10th-11th centuries, in Italy by Leonardo da Vinci, 15th—16th centuries, in Denmark by N. Steno, 18th century—and more systematically from the middle of the 18th century—for instance by M. V. Lomonosov in Russia and Hutton in Great Britain. Karl Hoff in Germany and particularly Charles Lyell in Great Britain made major contributions to the foundation and development of the method in the first half of the 19th century. The latter, however, having demonstrated the significance of actualism in the interpretation of geological history, used actualism as one of the elements of uniformitarianism, which arises from the concept of the immutability of the system of geological factors in time.

The error of these inferences was soon discovered, and since the end of the 19th century actualism has been applied to the study of the evolution of the earth and the constantly changing geological environment. Great contributions to the development of actualism have been made by Russian scientists like N. I. Andrusov, A. D. Arkhangelskii, and N. M. Strakhov, J. Walther in Germany, L. Cayeux in France, and others who confirmed the validity of actualism as a method and not as the fundamental principle of all geology.

Specialized discussions on the validity of using the term actualism, particularly in lithology, took place in Germany during the 1930’s and in the USSR during the 1950’s.