This article analyzes the heuristic method “One–variable–at-a-time”, which is useful in solving various problems of science and daily life, and presents characteristic applications of it in various fields of human activities. Furthermore, the method is used to explain logical paradoxes as well as the logical fallacy of the two variables. Finally, a variation of the method is presented, which can be characterized as its mirror opposite and is used, under special conditions, in problems of many variables where the original method cannot be used.