Sri Aurobindo translates Katha Upanishad, Second Cycle: Third Chapter, Verse 9: “God has not set His body within the ken of seeing, neither does any man with the eye behold Him, but to the heart and the mind and the super-mind He is manifest. Who know Him are the immortals.”
The physical senses perceive external forms. We fixate on the form and not the essence or content implied behind or within the form. The eye stops with the form and cannot therefore see the inner reality that exists. It is a tool of receiving impulses, not a tool of understanding the implications of what it experiences.
The Upanishad therefore indicates that it is not the physical senses that can recognize God, but to the interpretive and understanding parts of the being that receive the sense perceptions and can add knowledge or understanding to the raw impulse reception, as well as gain knowledge directly through faculties and standpoints not directly reliant on the sense organs. Thus, the “heart” is referenced. Spiritual experience as declared in the Upanishads repeatedly has the seat of the Atman, the soul, behind the heart. The “heart” provides relationship and a directness of understanding. The “heart” can know the reality through a sense of identity and oneness that it brings to the interaction, as the soul is the being of the Divine consciousness manifested in the individual.
The mind can go beyond the raw sense impressions as well, although it has its own limitations. The super-mind is the next level or power of knowing beyond the mind and it is able to both see the external forms and their purpose, as well as the inner reality that causes and is embodied through those forms. Each successive level of attainment moves the being away from external forms of knowing reliant on sense impressions and the interpretation of those impressions, toward “knowledge by identity” which cuts through the impressions of the outer forms. The highest form of knowing comes through this knowledge by identity, which is the status of those who are called “Immortals” because they have shifted from the ego-based consciousness of the individual focused on outer forms to the divine standpoint where they no longer are troubled by birth, life and death as traumatic events of an individual, but experience the process through the universal manifestation and the transcendence of Time-based experience.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129