Eliphas Levi records this concerning the Athanor:
"The ancient Magi were priests and kings, and the Savior's advent was proclaimed to them by a star. This star was the magical Pentagram, having a sacred letter at each point. It is the symbol of intelligence which rules by the unity of force over the four elementary potencies; it is the Pentagram of the Magi, the Blazing Star of the Children of Hiram, the prototype of equilibrated light. Towards each of its points a beam of light ascends, and from each a beam goes forth; it represents the Grand and Supreme Athanor of Nature, which is the body of man."1
The quotation clearly suggests the human body to be the alchemists' Athanor. In the same book, on page 115, Levi once again describes this Athanor in detail but forgoes naming it outright as "...it is too easy to divine." The two quotations should leave little doubt in anyone's mind that what is meant is the human organism. Never is he more accurate than when in the above quotation, he calls it "the prototype of equilibrated light." The Hebrew for Athanor is את הנור, ath ha-nur, meaning 'with the light.' All manifest creation is made of electro-magnetic radiant energy, the Limitless Light, but only generic humanity can say with accuracy that we are ‘with the light’. We are the only ones aware of that ‘certain something’ within ourselves, called any number of things through the ages, and which still defies description. Occultists and sages, however, have always known it to be AIN SUPh AUR, the one Limitless Light, the primal substance of God and Nature. Levi also compared the human body to an upright pentagram with a sacred letter and element assigned each of its points. These letters and elements are alluded to in many books on Tarot, Magic and Qabalah and need not be parroted here. However, the implication of a star with elements and the human body infers something more. With the elemental hints in mind, meditation on two Tarot Keys, The World and The Wheel of Fortune, bring to light the complete solution to the riddle of the "Quadrature of the Circle," but it all occurs inside the Athanor. Refer to AURUM POTABILE and LIVING STONE.
1 Levi, Eliphas, Transcendental Magic. New York: Samuel Weiser, 1972 ed., pg. 200.