Signs are a dynamic process transforming the input stimulus into a response, but what is the basis for this assumption? Why are there three types of sign relations, and why can they be classified into three classes? We find answers to these questions in Peirce’s category theory, his classification of phenomena. In addition, we show that Peirce’s categories explain the possibility of unified information processing. Proving that categories and the nine sign relation classes are practical, we discuss evidence of their use in various areas of human knowledge, including language processing, professional integrative negotiation, and mathematical problem-solving by elementary school children.