In the context of the organization of the brain, thinking and planning are mediated by the of the brain.

Cerebral cortex

In the context of the organization of the brain, thinking and planning are mediated by the cerebral cortex of the brain.  

The cerebral cortex is the most anterior and largest part of the cerebrum in the human brain. The cerebral cortex contains about 20 billion neurons which are involved in high levels of mental functioning. The cerebral cortex performs an important role in planning, memory, thinking, motor function, cognition, language processing, thought, and consciousness.

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