Why do different intellectual abilities tend to cluster together? Do we think in language? Why are human beings so intelligent? ‘Reasoning’ considers these questions by looking at the processes involved in non-verbal problem solving, spatial relations, thought, and language. Fluid reasoning, needed for IQ tests, depends on the dorsal executive system. Reasoning with verbal material involves the semantic system and also engages the parietal cortex. One reason for human intelligence is that humans are taught in a language, meaning that they inherit knowledge and understanding cumulatively. This is possible because of the expansion of non-primary areas in the human brain: the language areas, semantic system, and dorsal executive system, and in the prefrontal cortex.