Trying to understand Brahman with our limited minds is a bit like trying to look at your own eye, with which you see everything. To know Brahman with our limited minds is akin to feeling color or feeling pain. To know Brahman, one must rely on a special, nonsensual kind of seeing, which can be accomplished through one of the methods of God-realization.
One can only know Brahman through a transcendent state of consciousness. In the Upanishads, the negative description of God as being neti-neti(literally “not this-not this”) is the method by which Brahman is explained, a method later adopted by the medieval Jewish philosopher Moses Mai-monides. In speaking of God, Maimonides claimed we can only explain what God isn't, not what he is. He called this the via negativa, or the “negative way.”
He who really knows must understand the ultimate nature of things, which is akin to knowing their causes. Such is the meaning of the following passage in the Upanishads.
Sanatkumara said: “When you attain well-being, then you can act effectively. Those who do not attain well-being, cannot act effectively.
Therefore you must understand the nature of well-being.” The young man said: “I seek the nature of well-being.” Sanatkumara said: “When you enjoy the beauty of the world, then you can attain well-being. Those who do not enjoy the beauty of the world, cannot attain well-being. Therefore you must understand the nature of beauty.” The young man said: “I seek the nature of beauty.”
Sanatkumara said: “When you discern the infinite in that which is beautiful, then you can enjoy beauty. Those who do not discern the infinite in that which is beautiful, cannot enjoy beauty. Therefore you must understand the infinite.” The young man said: “I seek the nature of infinity.”
Sanatkumara said: “When you understand the indivisible unity of all beings, and see and hear nothing, you discern the infinite. When you see or hear only separateness and division, you discern only that which is finite. The infinite is beyond death, but the finite cannot escape death.”
— Chandogya Upanishad 7:21; 22; 23; 24.10
To understand the indivisible unity of things is, in the words of the last passage, to understand Brahman.