The aim of the paper is an extension of the idealizational theory of science in order to explicate intuitions of historians and philosophers of history about unpredictability and contingency of history. The author identifies two types of essential structures: the first kind dominated by the main factor and the second kind which is dominated by a class of secondary factors. In an essential structure dominated by the main factor, the power of influence it exerts is greater than the sum of the power of influence of secondary factors. In an essential structure dominated by secondary factors, their total influence is greater than the influence exerted by the main factor, although the power of the latter influence is greater than the power of influence of each secondary factor taken separately. In the latter kind of essential structure the so-called cascade effect can occur which means that at certain moment the power of influence of secondary factors can be greater than influence of main factor. In the further part of this paper, the author examines consequences of cascade effect for construction of scientific theory and historical narrative.