Harmony assumes the existence of multiplicity. But, as a matter of fact, existence is a unified whole: there are no two, it is all One. As the Katha Upanishad says, by the mind alone is the Supreme to be comprehended; there is no variety here. Whoever perceives variety here goes from death to death.
Explaining this verse, Swami Ranganathanandaji (Ramakrishna Mission) says:
Such self-realization steadily breaks down the barrier between man and man. This basic truth of non-separateness is to be realized by the mind not by the sense-bound mind, which is impure, but by the sense-free mind, which is pure. In human life, individual and collective, the stress of separateness has been the one source of hatred, violence, and war. It is through a purification of human knowledge and awareness that man transcends this false view of separateness and over comes its evil effects. By saying manasaivedam aptavyam, the Upanishad emphasizes the need for the right training of the mind. It emphasizes that this truth must come to us through the educational process right from child hood. Wrong conditioning of children, which instills into them false ideas of inferiority and superiority based on caste, race, sex, nationality, or religious differences, has done immense harm in the past
What should be the outlook of individuals in any cultured society? The answer can be found in a popular verse: This person is mine, that person is another’s this is the attitude of the small-minded. For the broad-minded the whole world is one’s own family (vasudhaiva kumbakam). Rightly does the Upanishad say, He who sees all beings in himself and himself in all beings, feels no hatred by virtue of that (realization).