muNDaka Upanishad
The Self is of the nature of pure sentience. Hence, it can be known only through that which is likewise sentient—namely, an entity belonging to the same genre. By way of illustration, a magnet attracts iron filings but not a piece of wood; for attraction is possible only between entities that share a common nature.
The body, the senses, and even the vital force (prANa), being insentient, are incapable of apprehending the Self. The mind (antaḥkaraNa) alone, being endowed with reflected consciousness and possessing the capacity for cognition, serves as the appropriate instrument for the knowledge of the Self. Therefore, the mind alone is competent to grasp It.
Further, the Self that is sought to be realized is utterly indivisible and without any internal distinctions. It is free from all forms of differentiation — whether internal, external, or relational.
A tree, though seemingly a single entity, is divisible into multiple constituent parts such as roots, trunk, branches, leaves, flowers, fruits, and seeds. It is thus a “divisible whole.” Space, on the other hand, is not subject to such division and may be regarded as an “indivisible whole.” The Self is akin to the latter.