Final answer:
Descartes believed the pineal gland was the 'seat of the soul' because he thought it was where the soul contacted the brain. However, scientific investigation has shown that the pineal gland does not function as a central control unit for the brain.
Step-by-step explanation:
Rene Descartes chose the pineal gland as the 'seat of the soul' because he believed it was the master control unit where the soul contacted the brain. He thought the pineal gland was unique compared to other parts of the brain because it was singular and not doubled. However, scientific investigation has since proven that the pineal gland does not function as a central control unit for the brain.