Human spirit and the rational soul

The human spirit which distinguishes man from the animal is the rational soul; and these two names—the human spirit and the rational soul—designate one thing. This spirit, which in the terminology of the philosophers is the rational soul, embraces all beings, and as far as human ability permits discovers the realities of things and becomes cognizant of their peculiarities and effects, and of the qualities and properties of beings.
But the human spirit, unless assisted by the spirit of faith, does not become acquainted with the divine secrets and the heavenly realities. It is like a mirror which, although clear, polished, and brilliant, is still in need of light. Until a ray of the sun reflects upon it, it cannot discover the heavenly secrets.
But the mind is the power of the human spirit. Spirit is the lamp; mind is the light which shines from the lamp. Spirit is the tree, and the mind is the fruit. Mind is the perfection of the spirit, and is its essential quality, as the sun’s rays are the essential necessity of the sun.

Selected Writings of Abdu’l-Bahá P.317