The German philosopher Arthur Schopenhauer read the Latin translation and praised the Upanishads in his main work, The World as Will and Representation (1819), as well as in his Parerga and Paralipomena (1851). He found his own philosophy was in accord with the Upanishads, which taught that the individual is a manifestation of the one basis of reality. For Schopenhauer, that fundamentally real underlying unity is what we know in ourselves as "will". Schopenhauer used to keep a copy of the Latin Oupnekhet by his side and commented,
It has been the solace of my life, it will be the solace of my death.
Another German philosopher, Friedrich Wilhelm Joseph Schelling, praised the mystical and spiritual aspects of the Upanishads. Schelling and other philosophers associated with German idealism were dissatisfied with Christianity as propagated by churches. They were fascinated with the Vedas and the Upanishads. In the United States, the group known as the Transcendentalists were influenced by the German idealists. These Americans, such as Emerson and Thoreau, were not satisfied with traditional Christian mythology and therefore embraced Schelling's interpretation of Kant's Transcendental idealism, as well as his celebration of the romantic, exotic, mystical aspect of the Upanishads. As a result of the influence of these writers, the Upanishads gained renown in Western countries.
One of the great English-language poets of the 20th century, T. S. Eliot, inspired by his reading of the Upanishads, based the final portion of his famous poem The Waste Land (1922) upon one of its verses. Erwin Schrödinger, the great quantum physicist said,
The multiplicity is only apparent. This is the doctrine of the Upanishads. And not of the Upanishads only. The mystical experience of the union with God regularly leads to this view, unless strong prejudices stand in the West.
Eknath Easwaran, in translating the Upanishads, articulates how they
...form snapshots of towering peaks of consciousness taken at various times by different observers and dispatched with just the barest kind of explanation.
Juan Mascaró states that the Upanishads represents for the Hindu approximately what the New Testament represents for the Christian, and that the message of the Upanishads can be summarized in the words, "the kingdom of God is within you".
Paul Deussen in his review of the Upanishads, states that the texts emphasize Brahman-Atman as something that can experienced, but not defined. This view of the soul and self are similar, states Deussen, to those found in the dialogues of Plato and elsewhere. The Upanishads insisted on oneness of soul, excluded all plurality, and therefore, all proximity in space, all succession in time, all interdependence as cause and effect, and all opposition as subject and object. Max Muller, in his review of the Upanishads, summarizes the lack of systematic philosophy and the central theme in the Upanishads as follows,
There is not what could be called a philosophical system in these Upanishads. They are, in the true sense of the word, guesses at truth, frequently contradicting each other, yet all tending in one direction. The key-note of the old Upanishads is "know thyself," but with a much deeper meaning than that of the γνῶθι σεαυτόν of the Delphic Oracle. The "know thyself" of the Upanishads means, know thy true self, that which underlines thine Ego, and find it and know it in the highest, the eternal Self, the One without a second, which underlies the whole world.—Max Muller