Ratio Christi is the “reason of Christ” (or the "rationality of Christ"). The Latin word ratio (pronounced "RA-she-oh"—the "A" is pronounced like the "a" in the English words "cat" and "rationality"; see note below*) can mean calculation, consideration, reason, and more. Indeed, Christianity is supported by probability calculations and other considerations drawn from human history (such as arguments for Jesus’ resurrection) and natural history (such as scientific arguments for God). The ancient Greek word logos has a range of meaning that overlaps with ratio. Logos can mean word, speech, argument, wisdom, and reason. Peter Kreeft has noted this in his book, The Philosophy of Jesus (2007), p. 9, Saint John even opens his biography of Jesus with: “In the beginning was the Logos and the Logos was with God and the Logos was God.” John tells us this Logos is Jesus. He is the source of all good things, including reason itself. Furthermore, Christian thinkers have long recognized that ratio and logos include discerning the inherent (God-given) intelligibility of any object of study. We can know a thing according to its nature, in view of its coherence and relations to other things, rather than just accidentally. This is possible because the creator, the ultimate ratio and logos, spoke the world into being. The world is designed to be known, and thus all academic disciplines are possible. This is ratio Christi--the reason of Christ.
Professor of the History and Philosophy of Science
The College at Southwestern (http://college.swbts.edu)
*Technical note on the Latin: This recommended "RA-she-oh" pronunciation of ratio follows a widely used method for Latin pronunciation in English context explained (here) by linguist Michael Covington who also offers alternate Latin pronunciation methods in this PDF document). Another common Latin pronunciation is RAH-tsee-oh, though this organization "official" pronunciation is RA-she-oh. English speakers routinely pronounce Christi like the Texan city of Corpus Christi (pronounced "KRIS-tee"), which means "the body of Christ."
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