Abu Suleyman al-Sijistani, a famous thinker in the classical age of Islam in logic and philosophical circle, had discussed methodological differences between philosophy and theology and their reflections on the problem of essence of God and talking about it. His style of handling the problem is quite unique in Islamic thought. If we say with a modern terminology, he developed a negative theology. For this reason, in this article we will discuss two things: First, the extent of differences between theology and philosophy; and secondly, how these differences impact al-Sijistani?s view on the essence of God and the way of talking about Him.