Vasubandhu (flourished 5th century ad) was an Indian Buddhist philosopher and logician, younger brother of the philosopher Asaṅga. His conversion from the Sarvāstivāda to the Mahāyāna Buddhist tradition is attributed to Asaṅga. Vasubandhu refined classical Indian syllogistic logic by distinguishing the procedure for reaching inferences in formal debate (five steps) from the method in personal thought (three steps). He wrote several śāstras (“treatises”) holding that all seemingly external objects are only mental representations, and he is also reputed to be the author of the Abhidharmakośa, a systematization of Sarvāstivāda doctrine written before his conversion.