Plotinos: Complete Works, v. 3 In Chronological Order,...

11. What then is the Principle which one cannot even say that it is (hypostatically) existent? This point will have to be conceded without discussion, however, for we cannot prosecute this inquiry. What indeed would we be seeking, when it is impossible to go beyond, every inquiry leading to some one principle, and ceasing there? Besides, all questions refer to one of four things: existence, quality, cause and essence. From the beings that follow Him, we conclude to the essence of the First, in that sense in which we say He exists. Seeking the cause of His existence, however, would amount to seeking an (ulterior) principle, and the Principle of all things cannot Himself have a principle. An effort to determine His quality would amount to seeking what accident inheres in Him in whom is nothing contingent; and there is still more clearly no possible inquiry as to His existence, as we have to grasp it the best we know how, striving not to attribute anything to Him.