Pascal’s father suffered an accident in 1646, which brought him into contact with a priest sympathetic to the ideas of Cornelis Jansen (1585-1638), the bishop of Ypres. Pascal also encountered Jansenist ideas on grace and piety, and was deeply affected by them, as were other members of his family. After her father’s death in 1651, Blaise’s sister Jacqueline became a nun at the convent at Port-Royal, which was the centre of Jansenist doctrine and religious practice in France.

Pascal opposed her vocation strongly, and indeed for two years thereafter led a life very different from hers, consorting with free-thinkers, gamblers and libertines of fashionable Parisian society; but on the night of 23 November 1654 he underwent a profound spiritual experience which altered his life irrevocably.

This ‘nuit de feu’ was such that Pascal sewed the record that he made of it at the time of the event itself as a permanent memento into his clothes, where it was found at his death. The document records an experience of conversion; not an intellectual experience, but one which persuaded Pascal of the superiority of instinctive belief. (This conviction was strengthened two years later, when his niece was cured miraculously at Port-Royal of an apparently incurable fistula.)

As a result of his experience, Pascal went to Port-Royal-des-Champs for a two-week retreat in 1655. There he met Isaac Le Maistre de Saci, a Jansenist theologian, with whom he had a debate, an account of which was published in 1720 as Entretien avec M. de Saci (A conversation with M. de Saci). This debate indicates not only that Pascal had conceived of an apology of the Christian religion in terms which would speak most powerfully to the very libertines and gamblers of the Parisian society which he had just forsaken, but also that he felt that one of the most powerful voices with which he had to contend was that of Michel de Montaigne, the gentleman philosopher who had championed the cause of scepticism in the later sixteenth century.

It is also clear that he felt the need to reassure those who had been shaken in their faith by new scientific developments, especially those in astronomy. Among Pascal’s immediate sources of inspiration was Antoine Arnauld, the leading Jansenist theologian and philosopher, who was at that time on the point of being condemned by the Sorbonne for his religious views. Together with Arnauld and Pierre Nicole, another prominent Jansenist, Pascal composed the Lettres provinciales (Provincial letters), a series of eighteen letters published in 1656-7.

These constitute a scathing attack on the moral and theological views of the Jesuits, who were the most vociferous opponents of both Arnauld and Jansenism. In this debate the Jesuits were somewhat unfairly represented as a religious faction which engaged in deliberate deception for political ends and sacrificed doctrine to morals, and Jansenists were for their part depicted as crypto-Calvinists whose interpretation of St Augustine was both erroneous and heretical. Pascal and his co-authors tried vigorously to rebut the charge of heresy levelled at Jansen’s writings while still acknowledging the authority of the Church that as Roman Catholics they were bound to accept. As it transpired, the debate was won de facto by the Jesuits; the Lettres provinciales were placed on the Index of Forbidden Books in 1657, and the Jansenist movement itself was condemned by the Pope shortly after. But the wit of the letters and their dazzling display of satire and irony have ensured that the judgment of posterity has been accorded to Pascal’s side, at least in literary terms.