He was challenged in his conclusions by Father Étienne Noel, the Jesuit Rector of the Collège de Clermont in Paris and a proponent of traditional Aristotelian physics; Pascal set out in his reply what are now taken to be the basic principles governing the application of scientific judgment and method. At the same time, he wrote to his brother-in-law Florin Périer to ask him to undertake the experiment of carrying a barometer up a mountain (the Puy-de-Dôme), the results of which showed that the level in the column of mercury varied with height. Pascal confirmed this himself on a church tower in Paris, and published the findings in 1648 in his Récit de la grande expérience de l’équilibre des liqueurs (Account of the great experiment on equilibrium in liquids); he concluded that experiment, not authority, governed physics, that his experiments had shown that nature has no horror of a vacuum, and that air pressure accounts for all the effects associated with said imaginary horror. These experimental writings played an important role in discrediting Aristotelian and scholastic scientific ideas

3 Pascal and Jansenism

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.