The origin of infallibility is one of the controversial topics under the topic of the infallibility of the prophets and pure imams (a.s.), and various theories based on different principles have been proposed in this regard. This article, regardless of the definition and scope of infallibility and focusing on practical infallibility and as a proof of infallibility, discusses and examines the origin of infallibility. At the beginning, it refers to the views raised about the origin of infallibility and discusses theories such as "knowledge and understanding", "love", "perfection of the intellect", "grace based on four causes", "will and choice", "knowledge and will", "knowledge and will and perfection of intellect". Then, while presenting Mulla Sadra's philosophy of action and referring to the principles of will which include imagination, confirmation, desire and emphasized desire, it presents a distinctive perspective in analyzing the origin of infallibility and similar views are criticized and examined. According to Mulla Sadra's philosophy of action and philosophical analysis of human will, the origin of infallibility is passion and love, but intense love that has overcome other loves. Knowledge, wisdom, and previous choices only affect human will when they cause love to arise and go through the process of intensifying and overcoming other loves. This article has collected information by referring to original theological sources and Mulla Sadra's works using a library method and then presented its final assessment by analyzing Mulla Sadra's philosophy of action.