Aristotle showed that every valid syllogism fit one of four basic patterns, although his followers in the Middle Ages claimed to discover more than four. Far more important, Aristotle showed (or seemed to show) that by linking one valid syllogism to another regarding a single subject, such as biology or ethics or even the nature of God, one could build a conceptual chain of reasoning that would inevitably lead, link by link, from one set of necessary truths to another, all the way to the highest truths of all.
