The Principle of Mentalism is the first of the seven Hermetic Principles, summed up in the axiom: "The All is Mind; the Universe is Mental." It holds that the whole of reality is, at root, mental, an idea in the mind of an underlying All.
A modern statement, an old name
These seven principles should be dated with care. They were set out in The Kybalion, published in Chicago in 1908 under the pseudonym "Three Initiates," a work generally attributed to the New Thought writer William Walker Atkinson. Although it invokes the name of Hermes Trismegistus and calls itself a Hermetic teaching, the seven-principle scheme is a product of the early twentieth-century New Thought movement, not of the ancient Hermetic writings such as the Corpus Hermeticum.
The principle is described here as part of that documented modern esoteric tradition, which has been widely influential in later occult and self-help literature.