Robert Indiana kept a series of illustrated journals during the late 1950s and 1960s, in which he discusses the development of his work as well as his daily life on Coenties Slip.

These two journal pages cover February 18, 1959, and February 18, 1960. Indiana's entry for February 18, 1959, appears on the top of the first page, and continues on the top of the second page. The entry includes a sketch of Source I, in a vertical orientation, with notes regarding the different colors used. Indiana describes working on the painting, both before and after he took a break to see the film He Who Must Die (1957) with Charles Hinman, who relayed having been fired from his job at the Cathedral of Saint John the Divine (a position previously held by Indiana). Indiana records trimming the edges of the Homasote to make a clean edge, improving the painting's surface, "almost sanded," and that "the major change was the narrowing of the white channel around the black core, and the increased size of the blue lacuna." He returned to the journal page the following day, February 19, adding the comment "It is very handsome now, particularly with the paint still wet."

The second entry, for February 18, 1960, begins on the bottom half of the first page, and continues on the bottom half of the second. Indiana does not discuss any works in this entry. He notes that he was met by a deluge of rain when he returned from the ferry, and records watching Ethan Frome (1960) and The Black Cat (1934) on television that evening.