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.

In this journal page, dated January 24, 1959, Indiana describes the completion of his painting Source I. A sketch of an egg, "The Source," is preceded by text noting that at this point he only had "the following shape in black arbitrarily painted in over the stomach; the womb; the middle regions of my only oil giant female." 

He explains that he began "to encloak" the egg form "within a shell, an investment of that inner vacuum or yolk: fluid be it whichever it is, and creating the crucial form of the painting which is yet, at noon, undecided." Indiana indicates that he had images of works by the artists Jack Youngerman, Kumi Sugai, and Jean Arp in mind, as well as the inevitable similarity to a phase of Robert Motherwell's work, but stresses staying true to his own organic form. He notes being "very excited, really agitated with its progress, which is really flowing today," with "divine" music (Sergei Prokofiev's Lieutenant Kijé and Beethoven's Fourth Symphony) on the radio, and the cold weather helping him feel directed and concentrated.

Indiana records completing the work by 4:30 p.m., and next to a sketch notes the colors (black, olive, cobalt blue, and white) used in each area of the painting. Although he acknowledges that the work, which was done on "a very rough oft-painted surface," is too crudely painted, he declares it "the start to what I must come to grips with in regard to color. The base color, in particular (olive) is my own, though the total effect is, here, too '[Ellsworth] Kelly' for comfort."