The French scientist/mathematician/philosopher Descartes argued that since we cannot trust our senses, we should only believe what can be known with certainty. He was able to satisfy his doubt first with an argument that he existed, and then that God existed.
Blaise Pascal rejected Descartes’s attempt to know God through reason.
Pascal was also a scientist/mathematician/philosopher. As a scientist he was instrumental in the invention of the barometer, and defender of the scientific method. As a mathematician he invented one of the first mechanical calculators. Descartes and Pascal ran in overlapping circles, and met on at least one occasion.
The Pensées were Pascal’s notes compiled posthumously by his family. Scholars in the last 50 years have taken special care to reorganize them into an order that they believe more accurately captures Pascal’s intention. Pascal’s fragmented notes convey many conflicting opinions because he was articulating the arguments of others so that he could supply his response to them, which often involved agreement on some points, and rejection of others.
Concerning Descartes’s method of doubt, Pascal agreed that it is rational for one to choose Christianity, but he rejected arguments for the existence of God. The Bible says God can only be known by those who pursue Him with all their heart. One cannot expect to prove or disprove Christianity with reason.
Since gambling was popular at the time, Pascal studied the mathematics behind it. He proposed that belief in the Christian God was rational if one saw it as a (forced) wager. The atheist’s demand for proof of the existence of God is irrelevant. One ought to be willing to sacrifice being right (concerning whether or not God exists) for the possibility of gaining infinite happiness. One is forced to take this wager. Pascal has framed the wager in such a way that it is irrational to take one’s mortality lightly. If the Christian is right that there is a God, when he/she dies he gains infinite happiness. If he/she is wrong, he loses nothing (except being wrong). By being right as an atheist one gains nothing (except being right).
The atheist say he/she may be convinced by Pascal’s argument, but is unable to make him or herself believe. Pascal offers a surprising solution. He says one can/may find belief if he/she lives life as a believer. The implication is that practice plays a determining role in one’s beliefs.