Introduction

St. Thomas was a philosopher in a particular historical context. He was born into a faith which he seems never to have doubted, and he was influenced, philosophically, by past philosophers as well as those who became his mentors and contemporaries. Among these the most important was Aristotle, but Aquinas kept his "free-born reason" intact throughout his many interactions with Aristotle. Introduced early to logic, Thomas takes for granted the familiar theorems or rules of sentential logic, including the basic theorems of normal modal sentential logics. Thomas was also familiar with Aristotelian views on syllogistic inferences. Like his contemporary, the logician Peter of Spain, Thomas agrees with Aristotle in treating universal affirmative and negative sentences (A and E sentences) as contraries – that is, they cannot be conjointly true, but they can be conjointly false. Thomas allows straightforward deductions of I sentences (particular affirmative) from the corresponding A sentences.