This phrase comes from the beginning of The Emerald Tablet and embraces the entire system of traditional and modern magic which was inscribed upon the tablet in cryptic wording by Hermes Trismegistus. The significance of this phrase is that it is believed to hold the key to all mysteries.

All systems of magic are claimed to function by this formula. "'That which is above is the same as that which is below'...Macrocosmos is the same as microcosmos. The universe is the same as God, God is the same as man, man is the same as the cell, the cell is the same as the atom, the atom is the same as...and so on, ad infinitum."

This message theorizes that man is the counterpart of God on earth; as God is man's counterpart in heaven. Therefore, it is a statement of an ancient belief that man's actions on earth parallel the actions of God in heaven. This pivots on the belief that "all things have their birth from this One Thing by adaptation."

To the magician the magical act, that of causing a transformation in a thing or things without any physical contact, is accomplished by an imaginative act accompanied by the will that the wanted change will occur.

The magical act and imaginative act becomes one and the same. The magician knows with certainty that for the change to occur he must will it to happen and firmly believe it will happen. Here it may be noted that magic and religion are akin: both require belief that a miracle will occur.

http://www.themystica.com/mystica/articles/a/below_above.html

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Fox.