A cognitive aspect on substitution in problem solving in mathematics

This is an article about the methodology of substitution in problem solving of mathematics. The operative use of substitution depends on two things: in an equation or situation in which variables appear, a substitution is a change of variable, and second, the change of variable is effective only if the change of variable makes the expression easier to understand and makes it possible to solve the problem. Substitution will be successful if the original equation has a symmetry or especial property that is possible to explore, and the proficiency in using the method of substitution depends on this.