Katha-1-3-5-yastvavijñā

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

This being so, of that driver known as Buddhi, who is not capable of discrimination as to what is to be done or omitted to be done, as the other driver in the guiding of his chariot has a mind like the reins of a chariot not well-grasped by the driver, i.e., uncontrolled by the intellect; of that incompetent driver, i.e., intellect, the senses which stand in the place of the horses become unruly and uncontrollable, as the vicious horses of the other driver.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda