The First Hermetic Principle is all about consciousness, stating that 'all is mind', putting consciousness over both matter and energy. The Principle of Mentalism implies a concept of natural, universal consciousness as the source and the essence of all being, as the creator and the creation, it is through consciousness that we - and all the beings in the universe - begin to exist as conscious ideas before we become physical realities.
Note that this concept is reflected in the procession from the Ace to the Ten in the Minor Arcana, from Kether to Malkuth in the Tree of Life. You will also find the 'Trinity' of Consciousness - Energy - and Matter.
As a sidepoint it should be said that this concept of an universal consciousness in which man is a part of the whole sharply collides with any idea of an omnipotent god or any other kind of divine ruler who would willfully manipulate the natural processes.
From the Kybalion:
This Principle embodies the truth that 'All is Mind.'
It explains that The All (which is the Substantial Reality underlying all the outward manifestations and appearances which we know under the terms of 'The Material Universe'; the 'Phenomena of Life', 'Matter', 'Energy', and, in short, all that is apparent to our material senses) is spirit which in itself is unknowable and undefinable, but which may be considered and thought of as an universal, infinite, living mind.
It also explains that all the phenomenal world or universe is simply a Mental Creation of The All, subject to the Laws of Created Things, and that the universe, as a whole, and in its parts or units, has its existence in the Mind of The All, in which Mind we 'live and move and have our being.'
This Principle, by establishing the Mental Nature of the Universe, easily explains all of the varied mental and psychic phenomena that occupy such a large portion of the public attention, and which, without such explanation, are non-understandable and defy scientific treatment.