Mental processes can be understood only if we understand the tools and signs that mediate them. In higher forms of human behavior, the individual actively modifies the stimulus situation as a part of the process of responding to it. All the higher functions originate as actual relations between individuals. For example, children may argue with each other about who gets to play with what toy, or how a task should be completed. The child converts social relations into psychological functions through mediation.
Mediation occurs through a linking tool or sign.
A tool is defined as something that can be used in the service of something else. But as adults, we can use language to regulate our behavior beyond our immediate needs or environment. For example, in the morning I may wake up and want a glass of orange juice, only to discover that I am out of orange juice. The diversity of symbols across cultures leads to differences in the kinds of mental functions that are developed. Thus, universal stages of psychological development across cultures cannot be identified.