Swami Sivananda Proves Existence of God by the study of mind and its functions

Unless there exists one continuous principle equally connected with the past, present
and future, or an absolutely unchangeable Self which cognizes everything, we are unable to account for remembrance, recognition and so on, which are subject to mental impression on place, time and cause. The Self is distinct from and superior to ideas, because the ideas require an ultimate principle which unites and connects them, while the Self is itself the ultimate Principle which renders the cognition of the ideas possible.

The Manas (mind) is an organ of sensation and thought. It must be under the control of some one who uses this instrument. The Jiva or human soul is not the director of the mind, because we see that ordinary men are swayed away ruthlessly by the mind. Therefore, there must exist some other Supreme Being, who is the Director of the mind. He is the Antaryamin, the inner Ruler and Controller.

Then again look at the miraculous power of the mind. In the Kenopanishad the first Mantra begins, 'Who is the Director of the mind?' There is the play of the Divine hand here also. Can my brother psychologist manufacture a mind from his laboratory? We are struck with wonder when we look into the diverse faculties of the mind, viz., power of discrimination, power of judgment,reasoning, retentive power, imagination, cogitative faculty or the power of reflection,understanding, etc. No one but God can create such a powerful and miraculous mind.

Source: GOD EXISTS By SRI SWAMI SIVANANDA