Know that the wayfarers who prove [His] attributes by [His] effects and by [His] attributes prove [His] essence follow different ways. The best of them are two. The first is to know the human soul: ‘And in your souls.Will you not then perceive?'(51:21). THis is the best way next to the way [proof] of the righteous (siddiqin). [Mulla Sadra, Divine Manifestations, Part One – Second Manifestation, pg 23, English version]
One should know that the soul from its physical conception to the end of its highest intellectual perfection is in constant change and alteration in its essence and continuous motion and revolution in its substance. Sometimes it is a physical power and some other times it is a natural form; sometimes it is a sensitive soul in its different degrees and sometimes it is imagining, recollecting and reflecting, and rational. It acquires theoretical intelligence after it gaining practical intelligence in different degrees, from potential intelligence to actual intelligence and the active intelligence that is called the ‘commanding spirit’ by Allah: ‘Say, The Spirit is of the command of my Lord’ (17:85). Evidently that which makes it leave the potential for the actual, and deficiency for perfection, must be absolutely actual in its essence, and its truth must be free from any potentiality to avoid the impossible regress.
On the other hand, that being should either be necessary in its being or one of the intellectual angels, those who ‘do not disobey whatever Allah has commanded them’ (66:6). Nevertheless, the existence of an intellectual immaterial being is inconceivable unless through the existence of the exalted Necessary Being. This, next to the Proof if the Righteous (burhan al siddiqin), is the best argument though there insufficient space to explain it in detail. [Hasan al Nuri, Ashtiyani, Khamene’i, commentary on above passage]