The Hermetic Principle of Mentalism is the first of the seven Hermetic Principles found in The Kybalion, a classic text that outlines the foundational teachings of Hermetic philosophy. This principle states that "The All is Mind; the Universe is Mental." In essence, it suggests that everything in existence originates from and is shaped by the mind, both on a macrocosmic (universal) and microcosmic (individual) level.

According to this principle, the universe and all its phenomena are mental constructs within the mind of The All—the infinite, universal consciousness or source from which everything emanates. It teaches that reality itself is a product of mental energy, and therefore, our thoughts and consciousness have the power to influence our experiences, manifestations, and perceptions of the world.

On a practical level, the Principle of Mentalism implies that we, as individuals, can exercise control over our lives by mastering our minds. By aligning our thoughts with higher truths, we can elevate our awareness, achieve self-mastery, and reshape our reality according to our will. This principle is central to many esoteric practices such as meditation, visualization, and manifestation, as it encourages practitioners to recognize the profound impact of mental energy on their external reality.

Ultimately, the Hermetic Principle of Mentalism emphasizes the interconnectedness of all things through consciousness and underscores the role of the mind as the foundational force of creation.