It is written in The Zohar, (Vayikra, Parashat Tazria, p 40), “Come and see, all that exists in the world, exists for man, and everything exists for him, as it is written, ‘Then the Lord God formed man,’ with a full name, as we have established, that he is the whole of everything and contains everything, and all that is Above and below, etc., is included in man.” Thus, it explains that all the worlds, Upper and lower, are included in man.And also, the whole of reality within those worlds is only for man.
Baal HaSulam, “Introduction to the Preface to the Wisdom of Kabbalah,” Item 1
Take our sense of sight, for example: we see a wide world before us, wondrously filled. But in fact, we see all that only in our own interior. In other words, there is a sort of a photographic machine in our hindbrain, which portrays everything that appears to us and nothing outside of us.
Baal HaSulam, “Preface to the Book of Zohar,” Item 34
For that, He has made for us there, in our brain, a kind of polished mirror that inverts everything seen there, so we will see it outside our brain, in front of our face. Yet, what we see outside us is not a real thing. Nonetheless, we should be so grateful to His Providence for having created that polished mirror in our brains, enabling us to see and perceive everything outside of us. This is because by that, He has given us the power to perceive everything with clear knowledge and attainment, and measure everything from within and from without.
Baal HaSulam, “Preface to the Book of Zohar,” Item 34
Even though we see everything as actually being in front of us, every reasonable person knows for certain that all that we see is only within our own brains. So are the souls: Although they see all the images in the Giver, they still have no doubt that all these are only in their own interior, and not at all in the Giver.
Baal HaSulam, “Preface to the Book of Zohar,” Item 34