Brain

A brain is the main central area for thought in most biological entities. The Rani constructed a giant brain to absorb the mental energies of various individuals and compute the computations for a time manipulator. (DW: Time and the Rani)

In certain circumstances the brain can be transplanted out of the biological entity into another vessel, as occured with Morbius' brain and the Cybus Industries Cybermen. (DW: The Brain of Morbius, Rise of the Cybermen)