On a September morning in 1647 (Louis XIV, aged nine, was king), a carriage drew up before the Paris house of the Pascal family, and M Rene Descartes got out. He looked like a crafty peasant. In fact, at 51, he was the most prominent mathematician-philosopher of his day. His famed Discourse divided French intellectuals into two camps - one either was or was not a "Cartesian".
Blaise Pascal, 24, was not. He had no argument with Descarte's axiom "I think, therefore I am", but he was less certain about the ability of reason to prove a) the existence of God or b) the non-existence of a vacuum in nature. Surely God was felt, not reasoned; and as for the vacuum - he had himself only recently conducted experiments that seemed to verify its existence. He was none the less pleased when Descartes asked to meet him, and, although Pascal was ill, a visit was arranged.
Also present were Professor Roberval, of the College de France, a voluble anti-Cartesian, and Pascal's younger sister Jacqueline. Pascal brought out a calculating machine, his recent invention, and demonstrated its ability to add and subtract. Descartes was impressed. The talk turned to the vacuum. Pascal described his experiment; Descartes expressed doubt - a polite skirmish that might have ended there. But Roberval injected his opinion, and a heated argument ensued. Descartes took his leave.
The next morning, however, he returned - not Descartes the philosopher this time, but Descartes the physician. He sat for three hours by his patient's side, listened to his complaints, examined him, prescribed soups and rest. When Pascal was sick of staying in bed, Descartes said, he would be nearly well. Their views would remain opposed, but it was the supreme rationalist in his role as kindly doctor whom Pascal would later remember, and who may have been in his mind when he observed, "The heart has its reasons which reason knows nothing of"
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