When it is not in our power to follow what is true, we ought to follow what is most probable.

This maxim is associated with Descartes’s practical “provisional morality” formulated while he pursued radical doubt and searched for indubitable foundations for knowledge. In the Discourse on the Method (1637), he explains that in ordinary life we often must act before certainty is available; therefore, while suspending judgment in theoretical inquiry, one should still adopt workable rules for conduct. One of these rules is to follow the most probable course when truth cannot be clearly known, and then to act with resolution rather than remain paralyzed by uncertainty. The remark reflects the tension in Descartes between demanding certainty in philosophy and accepting probability in action.

The sentence draws a pragmatic boundary around Descartes’s famous quest for certainty. It acknowledges that human agents often lack the power—time, evidence, or cognitive access—to determine what is strictly true. In such cases, rationality shifts from demonstration to probability: we should follow what is best supported by available reasons, even if it falls short of indubitable knowledge. The quote thus complements Descartes’s methodic doubt by preventing paralysis; it implies a two-tier epistemology in which theoretical inquiry aims at certainty, but practical life is governed by well-justified likelihoods.