text: did descartes really said that pascal has too much vacuum in his head?   yes.  since 1646 pascal began a series of experiments on atmospheric pressure. by 1647 he had proved to his satisfaction that a vacuum existed. descartes visited pascal on 23 september. his visit only lasted two days and the two argued about the vacuum which descartes did not believe in. descartes wrote, rather cruelly, in a letter to huygens after this visit that pascal had too much vacuum in his head.  in august of 1648 pascal observed that the pressure of the atmosphere decreases with height and deduced that a vacuum existed above the atmosphere. descartes wrote to carcavi in june 1647 about pascal's experiments saying: "it was i who two years ago advised him to do it, for although i have not performed it myself, i did not doubt of its succes"  in october 1647 pascal wrote new experiments concerning vacuums which led to disputes with a number of scientists who, like descartes, did not believe in a vacuum.  rene descartes (1596-1650)  blaise pascal (1623-1662)