The Theory of Deduction in Avicenna's Logic: An Aristotelian-Stoic Combination with Ptolemaic Errors

One can see the traces of propositional logic in Aristoteles''s works. However he admired categorical syllogism and though considered conditional syllogism as unreducible، considered them dependent on it. After Aristotle، his pupils tried to develop his theory of conditional syllogism، but they did not succeed. This was Stoics whose special approach to conditional proposition logic led them to establish the first system of propositional logic. Under the light of mathematical teachings of logic، the basic status of Stoic logic was found in the beginning of the twentieth century. Although Avicenna tried to apply both theories of Aristotle and stoics in his deduction theory، he considered Stoic theory unimportant and dedicated his attempts to develop the theory of «fully conditional syllogism» of Theophrastus. Belittling the Stoic''s theory is one of Avicenna''s methodological errors which are rooted in works of Alexander of Aphrodisias who had reported Stoic logic contemptuously. In addition، Avicenna considered categorical syllogism superior than the conditional syllogism in the second place and exceptive syllogistic in third in syllogism logic. That was his second methodological error which is rooted in Prior Analytics of Aristotle; that''s why this error is called as «Ptolemaic error».