Unlike modern readers, who follow the flow of a narrative from beginning to end, early modern Englishmen read in fits and starts and jumped from book to book. They broke texts into fragments and assembled them into new patterns by transcribing them in different sections of their notebooks. Then they reread the copies and rearranged the patterns while adding more excerpts. Reading and writing were therefore inseparable activities. They belonged to a continuous effort to make sense of things, for the world was full of signs: you could read your way through it; and by keeping an account of your readings, you made a book of your own, one stamped with your personality.
Just about anyone with intellectual ambition in the seventeenth and eighteenth centuries was likely to keep a commonplace book. In its most customary form, “commonplacing,” as it was called, involved transcribing interesting or inspirational passages from one’s reading, assembling a personalized encyclopedia of quotations. ... The great minds of the period—Milton, Bacon, Locke—were zealous believers in the memory-enhancing powers of the commonplace book. There is a distinct self-help quality to the early descriptions of commonplacing’s virtues: in the words of one advocate, maintaining the books enabled one to “lay up a fund of knowledge, from which we may at all times select what is useful in the several pursuits of life.”
Steven Berlin Johnson, "The Glass Box and the Commonplace Book," Hearst New Media lecture, Columbia University, April 22, 2010
Yet it is of Service to have Collections of this Kind, both that Students may learn the Art of putting Things in Order, as also the better retain what they Read.
John Locke, A New Method of Making Common-Place Books (1706); as quoted in "Taking Note: What Commonplace Books Can Teach Us about Our Past",Ever Note posted 26 Feb 2016.
When I meet with any thing, that I think fit to put into my common-place-book, I first find a proper head. Suppose for example that the head be EPISTOLA, I look unto the index for the first letter and the following vowel which in this instance are E. i. if in the space marked E. i. there is any number that directs me to the page designed for words that begin with an E and whose first vowel after the initial letter is I, I must then write under the word Epistola in that page what I have to remark.