The Systematic Ambiguity of Existence A concomitant of Mulla Sadra's theory that reality and existence are identical is that existence is one but graded in intensity; to this he gave the name tashkik al-wujud, which has been usefully translated as the 'systematic ambiguity' of existence. Al-Suhrawardi, in contrast to the peripatetics, had asserted that quiddities were capable of a range of intensities; for example, when a color, such as blue, intensifies it is not a new species of 'blueness' which replaces the old one, but is rather the same 'blue' intensified. Mulla Sadra adopted this theory but replaced quiddity with existence, which was for him the only reality. This enabled him to say that it is the same existence which occurs in all things, but that existential instances differ in terms of 'priority and posteriority, perfection and imperfection, strength and weakness' (making reality similar to al-Suhrawardi's Light). He was thus able to explain that it was existence and existence alone which had the property of combining 'unity in multiplicity, and multiplicity in unity.

Reality is therefore pure existence, but an existence which manifests itself in different modes, and it is these modes which present themselves in the mind as quiddities. Even the term 'in the mind', however, is merely an expression denoting a particular mode of being, that of mental existence (al-wujud al-dhihni), albeit an extremely attenuated mode. Everything is thus comprehended by existence, even 'nothingness', which must on being conceived assume the most meager portion of existence in order to become a mental existent. When reality (or rather a mode of existence) presents itself to the mind, the mind abstracts a quiddity from it - being unable, except in exceptional circumstances, to grasp existence intuitively - and in the mind the quiddity becomes, as it were, the reality and existence the accident. However, this 'existence' which the mind predicates of the quiddity is itself merely a notion or concept, one of the secondary intelligibles. It is this which is the most universal and most self-evident concept to which the Aristotelians referred, and which al-Suhrawardi regarded as univocal. But in reality there are not two 'things', existence and quiddity, only existence - not the concept, but the reality - and so 'existence' cannot be regarded as a real attribute of quiddity; for if this were possible quiddity would have to be regarded as already existent, as al-Suhrawardi had objected.

Substantial Motion Another of the key properties of existence for Mulla Sadra is its transubstantiality, effected through what he termed motion in substance (al-haraka fi'l-jawhar) or substantial motion (al-haraka al-jawhariyya). The peripatetics had held that substance only changes suddenly, from one substance to another or from one instant to another, in generation and corruption (and therefore only in the sublunar world), and that gradual motion is confined to the accidents (quantity, quality, place). They also held that the continuity of movement is something only in the mind, which strings together a potentially infinite series of infinitesimal changes - rather in the fashion of a film - to produce the illusion of movement, although time as an extension is a true part of our experience. What gives rise to movement is an unchanging substrate, part of the essence of which is that it is at an indefinite point in space at some instant in time; in other words, movement is potential in it and is that through which it becomes actual. Mulla Sadra completely rejected this, on the grounds that the reality of this substance, its being, must itself be in motion, for the net result of the peripatetic view is merely a static conglomeration of spatio-temporal events. The movement from potentiality to actuality of a thing is in fact the abstract notion in the mind, while material being itself is in a constant state of flux perpetually undergoing substantial change. Moreover, this substantial change is a property not only of sublunary elemental beings (those composed of earth, water, air and fire) but of celestial beings as well. Mulla Sadra likened the difference between these two understandings of movement to the difference between the abstracted, derivative notion of existence and the existence which is reality itself.

Existence in Mulla Sadra's philosophical system, as has been seen, is characterized by systematic ambiguity (tashkik), being given its systematic character by substantial motion, which is always in one direction towards perfection. In other words, existence can be conceived of as a continual unfolding of existence, which is thus a single whole with a constantly evolving internal dynamic. What gives things their identities are the imagined essences which we abstract from the modes of existence, while the reality is ever-changing; it is only when crucial points are reached that we perceive this change and new essences are formed in our minds, although change has been continually going on. Time is the measure of this process of renewal, and is not an independent entity such that events take place within it, but rather is a dimension exactly like the three spatial dimensions: the physical world is a spatio-temporal continuum.

All of this permits Mulla Sadra to give an original solution to the problem which has continually pitted philosophers against theologians in Islam, that of the eternity of the world. In his system, the world is eternal as a continual process of the unfolding of existence, but since existence is in a constant state of flux due to its continuous substantial change, every new manifestation of existence in the world emerges in time. The world - that is, every spatio-temporal event from the highest heaven downwards - is thus temporally originated, although as a whole the world is also eternal in the sense that it has no beginning or end, since time is not something existing independently within which the world in turn exists.