Knowledge is an evident and indefinable concept. According to Amir al-mu'minin, knowledge is possible and desirable, and it is the ultimate limit of all virtues, and man needs it in all his works. Exploration of the words of Amir al-mu'minin in sources such as Nahj al-Balaghah and Gharar al-Hakam leads the researcher to substitute and coexist terms of science and knowledge. In Amir al-mu'minin's viewpoint, knowledge is possible, and with different epistemological tools, one can achieve its various degrees. Thus, the possibility of knowledge is constant and beyond material matters. It is achievable through multiple means such as sense, intellect, and intuition. With a library method and analytical approach, the present article explores some of the essential epistemological issues through the words of Amir al-mu'minin. Therefore, after defining knowledge, the possibility of knowledge and its types have been studied, then authors talked about the sources of knowledge and their epistemological value.