[Line 1] RAHERE. A courtier of King Henry I (the youngest son of William the Conqueror) who ruled England from 1100 to 1135. According to legend he was the King’s jester — as here and in "The Tree of Justice" in Rewards and Fairies. He founded St Bartholomew’s Hospital, doing so, according to legend, after seeing a family of lepers in a London street.
[Line 3] Feed. Paid fees (given money for his services).
[Line7] St Michael’s. St Michael’s Mount, a lofty pyramidal island rising 400 yards from the shore of Mount’s Bay in Cornwall, in the west of England. It is united with the mainland by a natural causeway passable only when the sea retreats at low tide. Rahere’s “evil mood” is likened to the expanse of barren sand revealed at ebb-tide.
[Line 9] a Horror of Great Darkness. In Genesis 15 a deep sleep and “an horror of great darkness” fall on Abram at sunset when God makes his covenant with him. Here it signifies deep depression.
[Line 11] Gilbert the Physician. Gilbertus Anglicus, medical writer and practitioner, author of a Compendium Medecinae which includes a section on leprosy. His exact dates are unknown, but he flourished around 1205 — some sixty years after Rahere’s death in 1144, which is too late for him to have been a contemporary of the latter, as Kipling makes him here.
[Lines 14-16] In mediaeval medicine, four bodily fluids or humours — bile or choler, black bile or melancholy, blood, and phlegm — were regarded as determining by their relative proportions a person’s health and temperament. According to Burton’s Anatomy of Melancholy (4th ed.,1632) spirit is “a most subtile vapour, which is expressed from the blood, and [is] the instrument of the soul, to perform all his actions; a common tie or medium, betwixt the body and the soul, as some will have it.” The phrase 'a humour of the Spirit' mixes up the two, but must mean a state of the spirit caused by one of the humours (black bile or melancholy). In this state, Gilbert says, the spirit abhors excess and labours to expel it, whatever the cause of the surfeit (another word for excess).
[Line 18] Wanhope. Despair.
[Line 20] it comes …it passes — to return. Clinical depression, if untreated, usually abates spontaneously in the course of time, but is apt to recur.
[Line 22] Smithfield. Smithfield, an open space near the City of London, was well known in the Middle Ages for its horse market and as the venue for Bartholomew Fair It was a place of public execution for over 400 years, and criminals were hanged there until the gallows were moved to Tyburn around 1400.
[Line 28] checked. Came to a halt.
[Line 30] A motion of the spirit. Note the contrast with “a humour of the spirit” in Line 14. Love, says Gilbert, arises from a movement of the Spirit, unlike despair which results from the operation on the Spirit of one of the body’s humours.
[Line 34] the Essence and the Substance are the same. In the scholastic philosophy of the Middle Ages the term Substance was applied to the abstract notion of matter as the substratum of all possible forms, while Essence was used to mean the qualities expressed in the definition of a thing which when embodied in substance make that thing what it is. In perfect love, says Gilbert, the soul experiences the two as a single entity.