'Life is a pure flame and we live by an invisible sun within us'.
Long, long before the singer Sting's hit song 'Invisible Sun' in 1981, the image of an invisible sun occurs in Renaissance alchemical literature. The above quote a can be found in the fifth and final chapter of Sir Thomas Browne's 1658 Discourse Urn-Burial. It became better-known when Penguin books printed it on the cover of a paperback edition of Sir Thomas Browne's Urn-Burial in 2006. The startlingly original image occurs in the fifth and final chapter of the apotheosis of the Discourse -
'But man is a Noble Animal, splendid in ashes, and pompous in the grave, solemnizing Nativities and Deaths with equal lustre, nor omitting Ceremonies of bravery, in the infamy of his nature. Life is a pure flame, and we live by an invisible Sun within us'.
Perhaps Sting once read of an 'invisible sun' which subsequently influenced the imagery of his lyrics ? Never underestimate the power of cryptomnesia, a convenient forgetfulness amongst many artists and poets!
But in fact it was the Belgian alchemist Gerhard Dorn (circa 1530-84) who using Paracelsian 'astral imagery' for his own purposes, was the first to claim that within man there is an 'invisible sun', that is, a life-giving force, equivalent to the imago Dei, or image of God within man.
The image of an 'invisible sun' can be found in Dorn's Speculativa philosophia, which was reprinted in the door-stop sized tomes of the alchemical anthology known as the Theatrum Chemicum (vol.1 1604) an edition of which was once in Sir Thomas Browne's library, and from where in all probability he 'borrowed' imagery of an invisible sun. 
C.G. Jung in his Mysterium Coniunctionis (1955-56) cites Dorn, who is in fact the most frequently quoted alchemical author by C.G. Jung. In his Speculativa Philosophia Dorn declares-
The sun is invisible in men, but visible in the world, yet both are of one and the same sun.
Carl Jung in his own magnum opus on alchemy Mysterium Coniunctionis (1955-56) cites Dorn's image, commenting-
In Dorn's view there is in man an 'invisible sun', which he identifies with the Archeus. This sun is identical with the 'sun in the earth'. The invisible sun enkindles an elemental fire which consumes man's substance and reduces his body to the prima materia. 
In any event the source of what is essentially an 'imago dei', (image of God) continues to attract interest, along with Browne's esoteric associations in general, the physician-philosopher paradoxically to modern sensibilities being equally deeply-immersed in Hermeticism, alchemy and astrology, as well as promoting the 'new science' and the Baconian investigation into nature's properties.
 1711 Sales Auction Catalogue page 25 no. 124
 C.G. Jung Mysterium Coniunctionis Paragraph 49