Common folk, who judge things according to their first appearance, attribute the uncertainty of outcomes to an uncertainty in the causes—they think that the causes often fail to have their usual influence even when they don’t meet with any obstacle to their operation. But philosophers and scientists, observing that almost every part of Nature contains a vast variety of mechanisms and forces that are hidden from us because they are so small or so distant, think it at least possible that the contrariety of outcomes may come not from any contingency [here = ‘unreliability’] in the cause but rather from the secret operation of contrary causes. This possibility becomes certainty when they bear in mind that when any contrariety of effects is studied carefully it always turns out that it does come from a contrariety of causes, and proceeds from their mutual hindrance and opposition. A peasant can give no better reason for a clock’s stopping than to say ‘It often doesn’t go right’; but a clockmaker easily sees that the same force in the spring or pendulum always has the same influence on the wheels, but has failed of its usual effect because of a grain of dust that puts a stop to the whole movement. Having observed various cases of this general kind, philosophers and scientists form a maxim that the connection between all causes and effects is equally necessary, and that its seeming unreliability in some cases comes from the secret opposition of contrary causes.