From the Complete Works of Swami Vivekananda – Volume 9 – Writings: Prose and Poems(Original and Translated)

[An undated and untitled, one-page manuscript in Swami Vivekananda’s own handwriting]
My nerves act on my brain—the brain sends back a reaction which, on the mental side, is this world.
Something—x—acts on the brain through the nerves, the reaction is this world.
Why not the x be also in the body—why outside?
Because we find the already created outside world (as the result of a previous reaction of the brain) acts on us calling on a further reaction.
Thus inside becomes outside and creates another action, which interior action created another reaction, which again becomes outside and again acts inside.
The only way of reconciling idealism and realism is to hold that one brain can be affected by the world created as reaction by another brain from inside, i.e., the mixture x + mind which one brain throws out can affect another, to which it’s similarly external.
Therefore as soon as we come within the influence of this hypnotic circle, or influence, created by hundreds of preceding brains we begin to feel this world as they see it.
Mind is only a phase of matter, i.e., of the ever-changing phenomena of which matter and mind are different states or views. There must be something in whose presence this eternal, phenomenal net is spread—that is the Substance, the Brahman.