Samuel Hartlib was the central node in the most extensive Protestant intellectual network of the seventeenth century. Born in Elbing (now Elbląg, Poland) to a Polish mother and German father, he settled in England in the 1620s and devoted his life to the promotion of what he called the ‘advancement of learning’: the systematic collection and dissemination of useful knowledge. His correspondence network encompassed natural philosophers, theologians, educational reformers, physicians, and projectors across England, the Dutch Republic, Germany, and Scandinavia. He acted as an informal publisher, circulating manuscripts in scribal form; as an advocate, lobbying Parliament for educational reform; and as a patron, supporting Comenius, Culpeper, and many others from his own limited resources. Francis Bacon’s vision of a House of Solomon in the New Atlantis was his model; the Royal Society was his institutional heir.

John Dury was a Scottish minister who spent four decades in pursuit of a single goal: the union of the Protestant confessions. His methods were diplomatic and epistolary; he travelled continuously across Protestant Europe, corresponded with virtually every significant figure in the Reformed tradition, and produced a stream of pamphlets and proposals. The ecclesiastical project was never realised, but Dury’s collaboration with Hartlib on educational reform was more productive: his educational writings, developed in correspondence with Comenius, argued for a curriculum grounded in practical utility and governed by the progressive reform of the learner’s understanding rather than the accumulation of received content. His connection to the Hartlib circle made him a significant figure in the prehistory of English educational theory.

Jan Amos Komenský — Comenius — was the most systematic educational theorist of the seventeenth century and the originator of the pansophic programme that the Invisible College sought to institutionalise. His Didactica Magna (Great Didactic), drafted in Czech in the 1620s and published in Latin in 1657, argued for a universal system of education available to all people regardless of sex, class, or nation, organised on the principle that the order of instruction should follow the order of nature. His Orbis Sensualium Pictus (The Visible World in Pictures, 1658) was the first illustrated children’s textbook; it remained in use for over a century. His pansophic project — the construction of a unified system of all human knowledge, from which error, superstition, and controversy would be systematically eliminated — was the intellectual ancestor of the Encyclopaedia. He spent his London visit of 1641–1642 writing A Reformation of Schooles at Hartlib’s invitation.

John Wilkins was the Warden of Wadham College, Oxford, whose rooms became the meeting place of the experimental philosophers who would found the Royal Society, and the author of the most ambitious work in seventeenth-century linguistics: An Essay towards a Real Character and a Philosophical Language (1668). This work attempted to construct a universal language in which words would directly represent ideas, organised in a systematic taxonomy of all concepts. The project was not successful as a practical language, but it represents the most rigorous attempt to apply the new philosophy of classification — drawing on Bacon, Descartes, and Ray’s botanical taxonomy — to the structure of human language. Wilkins also wrote early works on cryptography, on the possibility of a voyage to the moon, and on the plurality of worlds.

Robert Boyle was the natural philosopher who did most to establish chemistry as a discipline distinct from alchemy and to articulate the methodological principles of the experimental programme. His Sceptical Chymist (1661) challenged both the Aristotelian four-element theory and the Paracelsian three-principle theory, arguing that neither was adequate to the results of careful experiment, and proposed a corpuscular theory of matter as the appropriate framework. His pneumatic experiments with the air pump, conducted with the assistance of Robert Hooke, produced the relation now known as Boyle’s Law and established the spring and weight of the air as central problems of natural philosophy. He also wrote extensively on the relationship between natural philosophy and religion, arguing that the investigation of nature was an act of piety that revealed the wisdom and power of the Creator.

Robert Hooke was the most versatile natural philosopher and instrument-maker of the seventeenth century, and the figure most responsible for the experimental programme of the early Royal Society as its Curator of Experiments. His Micrographia (1665) was the first major work of microscopic investigation, combining detailed engravings of insects, plants, and materials with theoretical discussion of light, colour, and the nature of living matter. He formulated the law of elasticity (Hooke’s Law), designed and built scientific instruments including a compound microscope and a Gregorian reflecting telescope, made significant contributions to the design of the balance spring for watches, and proposed an early version of the inverse-square law of gravitation. His relationship with Newton, his most serious rival, was one of the most bitter priority disputes in the history of science.

Katherine Jones, Viscountess Ranelagh, was Robert Boyle’s eldest sister and one of the most intellectually significant women in seventeenth-century England, operating at the intersection of the Hartlib circle, the parliamentary political world, and the experimental philosophical community. Her London house in Pall Mall was a salon and meeting place for natural philosophers, theologians, and parliamentarians; she corresponded extensively with Hartlib and was closely engaged with the practical and medical dimensions of the reform programme. She maintained a laboratory and was knowledgeable in chemistry and medicine. Her intellectual importance was substantial but largely invisible in the published record of the period, exercised through correspondence, conversation, and the management of networks rather than through print.