A Paribbājaka. He once visited the Buddha at Veluvana and questioned him regarding predestination. The Buddha explains to him that suffering arises from various causes -
and to say that these are all predestined is to go too far. Sīvaka expresses his approval and declares himself the Buddha's follower (S.iv.230f.; this Sutta is quoted at Mil. 137). Another conversation he had with the Buddha is recorded in the Anguttara Nikāya (A.iii.356). There he asks the Buddha if the claims made with regard to the Dhamma are justified. The Buddha proves to him, by illustration, that they are.
Buddhaghosa explains (SA.iii.87) that the Paribbājaka's name was Sīvaka, his sobriquet being due to his having worn his hair in a topknot.