Swift, in his “A Letter of Advice to a Young Poet” suggests that
A commonplace book is what a provident poet cannot subsist without, for this proverbial reason, that “great wits have short memories:” and whereas, on the other hand, poets, being liars by profession, ought to have good memories; to reconcile these, a book of this sort, is in the nature of a supplemental memory, or a record of what occurs remarkable in every day’s reading or conversation. There you enter not only your own original thoughts, (which, a hundred to one, are few and insignificant) but such of other men as you think fit to make your own, by entering them there. For, take this for a rule, when an author is in your books, you have the same demand upon him for his wit, as a merchant has for your money, when you are in his.
Typically these books were compilations of brief passages, often with commentary, ordered topically or thematically—in short they were collections of commonplaces—or, for those with the Greek tongue, koinoi topoi, or loci communes, in the Latin .
The sig-o-matic got me thinking of the old notion of the commonplace book, that portable and personal trove of epigrams, thoughts, and bromides that people used to treasure. The idea was that you read with an eye to noting for your own later use passages that resonated with you: these would be carefully transcribed in a blank book of your own, perhaps with your own observations, perhaps just as a collection of the wit and wisdom of others. Weblogs are an analog of this idea and the sig-o-matic (readers are invited to convert their favorite works in .sig entries) is a way of sharing your “commonplaces” with others.