Rationalism is absolutely indispensable when seeking a Science Victory, and is also quite helpful for Domination Victory seekers who want to field cutting-edge military units. Be aware that in Rise and Fall and Gathering Storm, only cities that meet the Population requirement and have high adjacency bonuses for their Campuses will fully enjoy the policy's Science bonus. The best civilization to make use of this card is the Maya, who can easily meet both conditions. To a lesser extent, Korea and Australia also love this card, since their Campuses are likely to have good adjacency bonuses, but they will have a harder time growing the Population of their cities to meet the second condition.
Rationalism is the worldview that looks to reason (rather than, say, blind faith or magic mushrooms) to explain the world. In this philosophy, reason trumps all other ways of knowing things, including any data gathered by one's five senses. Its roots can be traced back to the great Socrates, but it enjoyed something of a resurgence following Christian repression during the 16th and 17th centuries during the Scientific Revolution. Scientist-philosophers such as Réne Descartes, labelled the “father of modern philosophy,” Spinoza, Leibniz, and Kant altered the way humans viewed their world in profound ways and made rationalism the new faith.