Source: The Learning of the Glorious Qur’an, by Ayatullah Misbah Yazdi page 82.
In manifesting these terms, the philosophers begin with the Unity of Essence. They say that we must first believe in the fact that there are no attributes added to the Essence, then we may realize that in His Acts Allah is in no need of help and helper. But the gnostic, explaining these subjects on the basis of the human procession, starts with the Unity of Acts, i.e. "man's procession and advance towards Allah." The first thing uncovered to him is the Unity of Acts, then, when he reaches a more perfect stage, he deserves to understand the Unity of Attributes. The last stage to which a gnostic arrives is the Unity of Essence, though not as the philosopher explains it.
At-Tawhid in Acts means, to them, that when a person gets his soul purified, he will see every act to be Allah's, and the others are merely means and instruments. The hand which, from behind the curtain of causes, directs the world, creates everything in time and puts everything in place, is the powerful hand of Allah - a hand which is present everywhere and in all times. The tiniest phenomenon that happens in the world is created by Allah. So, what is the role of the material causes? They are no more than instruments, such as the incomplete simile of a pen in the hand of a writer.
The writer writes with the pen, but the basic role is played by the writer himself. The gnostic believes that, having believed in Allah and made it his profession to obey and worship Him, man would receive such a light from Allah that, he would be able to see and find the world's phenomena as they are, not just to understand them. But the Unity of Acts is one of at-Tawhid's degrees which man can attain first. We can also, more or less, attain them, through the philosophical studies, and by means of knowledge. But they (gnostics) see (witness) and find the facts without need of learning them. There is a famous story which clearly shows this fact but, we don't know to what extent it is right.
It is related that there was a meeting between Avicenna, (Ibn Sina), a philosopher, and Abu Said Abul-Khayr, a gnostic. "How did you find Abu Sa'id Abul-Khayr?" the students of Avicenna asked him. "He sees whatever I know", was his reply. Asked by his students about Avicenna, Abu Said Abul-Khayr said: "He knows whatever I see". Now, suppose that this story is untrue, but it quite clearly shows the basic difference between philosophy and gnosticism as an important one. The object of philosophy is to know, and the result of gnosticism is seeing and finding - that is, the real perfect gnosticism. So, this is the first stage which man reaches on his procession towards the stages of at-Tawhid. Having passed the stage of knowing himself, and stepped onto the valley of at-Tawhid, the first stage of at-Tawhid which man attains to and sees, is at-Tawhid in Acts (Unity of Acts).