Final answer:
Descartes chose the pineal gland as the 'seat of the soul' because he believed it was the central control unit where the soul interacted with the brain and body.
Step-by-step explanation:
The reason Descartes chose the pineal gland as the 'seat of the soul' was because he believed it was the central control unit where the soul interacted with the brain and body. He thought that the pineal gland, being a singular structure in the brain, was not duplicated like other parts of the brain. However, modern scientific investigation has shown that the pineal gland does not function as a central control unit for the brain.