The Corpus Hermeticum's opening treatise, the Poimandres, presents a visionary account in which the narrator is addressed by a divine figure identifying itself as Nous, Mind, who declares itself the source of light and life from which all things, including the physical cosmos itself, come into being, establishing mind, rather than any prior material substrate, as the most fundamental reality from which everything else derives.
The Kybalion, published in 1908 and presenting itself as a distillation of ancient Hermetic teaching, though modern scholarship generally treats it as a genuinely early twentieth century New Thought composition rather than a direct transmission of ancient doctrine, made this claim the first of its seven Hermetic principles in its own systematic restatement, that the All is Mind, the universe is mental, from which its other six principles, correspondence among them, are then derived as consequences: if the universe is fundamentally mental, then a mind operating according to definite mental laws is precisely the kind of thing that would produce the ordered correspondences the doctrine of correspondence describes between higher and lower levels of reality. This gives mentalism a distinctive relationship to idealism as understood in Western academic philosophy: the two claims are structurally close, both holding mind more fundamental than matter, but mentalism's Hermetic and New Thought lineage frames the claim in terms of a universal, largely impersonal cosmic Mind that the individual mind can align itself with and draw on, a practical, almost technical emphasis considerably more pronounced than in Berkeley's or Hegel's academic idealism.