The human intellect deals with universals: they are intelligible notions that correspond to their particular instances in the external world. The same applies to number abstracted from the things numbered, to quantity abstracted from what is measured, to quiddity abstracted from existence, to time abstracted from motion, to place abstracted from body, and to the accidents that attach to all of these.
For this reason the furthest point reached by their most exacting thinkers is what they call “absolute existence,” meaning the existence common to all beings. Such absoluteness, however, is found only in the mind, not in concrete reality, yet the philosophers make this shared universal the very subject of divine science.
As for the “Necessary Existence,” they put forward several definitions. At times they describe it as existence restricted by purely negative predicates, as Ibn Sīnā does. At other times they regard it as existence stripped of every negative and positive qualification, as claimed by certain atheists among the esoteric Rāfiḍites and the mystical unionists. Still others equate it with the very existence of all beings, denying the contingents any reality apart from the Necessary Existence itself. A full treatment of their claims has been given elsewhere.
Their ultimate end is that they conjure up something within themselves and then imagine that it exists outside them. It is here that the devils aid them; for devils manipulate the imagination and cast into people’s minds notions that possess no reality. Indeed, the more “rigorous” among these philosophers go so far as to say, “The realm of reality is the realm of imagination.”
On the established, witnessed existence of unseen matters: The unseen realities of which the Messengers—peace be upon them—have informed us are existent, firm, and far greater and more perfect than anything we observe in this world. They are tangible, perceptible realities that will be seen and felt after death and in the Hereafter, though God may allow certain chosen individuals to witness them even in this life. They are not mere mental constructs subsisting in the intellect, and this marks the essential difference between them and ordinary sensible objects.