It is difficult to define the Hartlib circle precisely. The Hartlib circle was not a society with a membership that gathered regularly, but a more diffuse group of individuals, widely dispersed geographically. From the late 1620s to the early 1660s, hundreds of different people met or corresponded with Samuel Hartlib in order to discuss points of educational theory, divinity or natural philosophy, and to suggest schemes for practical improvement. Hartlib was assiduous in seeking out new contacts and informants, often through intermediaries. Much of the work which he recorded was done by individuals whom he never met and similarly Hartlib's own reputation was spread by publication and correspondence well beyond the circle of his immediate friends and acquaintances.
The limited selection of published books and pamphlets represented here cannot capture the full variety of Hartlib's friends and correspondents, or of their activities. What it does illustrate, however, is the broader discussion taking place within the Hartlib circle around the theme of improvement. Such discussion was often inspired by scripture, and branched out to include political and religious as well as practical matters. The authors or editors of the books and pamphlets represented also played an important role in some part of Hartlib's career and helped him to develop his ideas about nature and society, benefiting in turn from his intellectual and, in some cases, practical assistance. Works by other members of the Hartlib circle can be found in the other sections of the catalogue.