All accounts of Hobbes's life are agreed that the years he spent in Paris, from late 1640 to the end of 1651, were of enormous importance to his philosophical development. They are also agreed that two people played particularly significant roles, as friends and intellectual influences: Marin Mersenne and Pierre Gassendi. Both were philosophers in their own right, and it is understandable that modern students of Hobbes should try to relate his ideas to the arguments set out in their published works. There is a danger, however, that this approach to Hobbes and his Parisian milieu may lead us to underestimate the importance of a third figure, merely on the grounds that he published almost no philosophical work: the mathematician Gilles Personne de Roberval. Enough evidence has survived to suggest that his personal and intellectual relations with Hobbes were almost as close as those enjoyed by Mersenne, and perhaps even closer than those of Gassendi. Regardless of how one might choose to rank the three in precise degrees of friendship with Hobbes or influence on him, the fact remains that Mersenne, Gassendi, and Roberval constituted a triumvirate of friends, the third of whom has been strangely neglected hitherto in all accounts of Hobbes's life.
Gilles Personne was born in the village of Roberval, near Senlis, in 1602. Nothing is known about his early education; his father was a poor farmer or farmworker, and the young mathematician (who would later add 'de Roberval' to his surname) seems to have led the peripatetic life of an impoverished student, passing through several universities and alternately studying and teaching. In 1628 he settled in Paris; there he got to know Mersenne, who recognized his talents and encouraged him to work on the problem of the curve known as the 'trochoid', 'roulette', or 'cycloid'. In 1632 Roberval was given a teaching post at the Collège de Maître Gervais; two years later he obtained a more eminent position, the Ramus chair of mathematics at the Collège Royal. 1 He would remain in this professorship