What the philosophers and the mystics mean by the ‘unity of existence’ is not that the whole universe put together is God; because, the whole collection does not enjoy a ‘real existence and unity’ (wujud wa wehdat-e-haqiqi). Similarly it also does not mean the ‘union’ (ittehad) of God with the creation, because union (in the sense that two things without losing their peculiarity and separate identity, transform into one thing) is rationally impossible (muhaal). In the same way, abandonment (tajafi) of one state (maqaam) and adoption (talabbus) of another is also not intended. Instead what it means is that ‘being’ (hasti) is confined in the Essence of Allah; ‘everything other than Allah’ (ma siwa Allah) is His appearance (namud), theophany (zuhur), and manifestation (tajalli)[a]; according to this elaboration, the intensity (shiddat) and perfection (kamal) of Allah’s existence, Who is the absolute being (hasti-e-mahdh), results in the passion for theophany and manifestation. The Essence (zaat) of the Almighty is the very love and passion, and the most adorable thing for the Almighty is the observation of His own Essence through the observation of [the glorious] Self-Essential affairs [or dimensions][b], which is termed as ‘istijlaa’’ (revelation of the Almighty’s Essence for Himself in ‘determinations’ (ta’ayyunaat)). And this complete ‘istijla’ (istijla-e-tamm) cannot be achieved but by His theophany in each and every one of the dimensions. In other words ‘unity of existence’ means that existence in the very state of being specific (shakhsi), its variety, multiplicity and different types and their effects are also a fact; like the human ‘self’ which is a singular unit, because definitely every individual is not more than one person, but at the same time, along with all his faculties; external or internal [intellectual (mashaer), digestive (hadhma), growth (namiya), and reproductive (muwallida)], is also united such that ‘self’ is the faculties itself and the faculties are the ‘self’ itself. Therefore ‘unity of existence’ according to this correct interpretation contradicts neither the continuous expansion of the universe nor the variety and multiplicity of existents.
While elaborating on this concept, a few points must be kept in mind:

In the philosophers’ view, the meaning and concept of ‘unity’ is axiomatic and does not need any definition because, they believe that unity is synonymous with existence, and defining it, just like the definition of existence, is not possible, without resulting in a vicious circle (daur), or definition of a thing by itself. [1]

The philosophers while mentioning the types of unity or ‘one’ [or unitary] (‘wahed’), say: A ‘one’ is either real (haqiqi) or, non-real (ghair-e-haqiqi). The ‘Real One’ (‘wahed-e-haqiqi’) is one who has unity as an attribute of his essence, and needs no mediator in the occurrence (‘urooz) of unity [for it], for example ‘one’ person. The ‘non-real one’ is something of which unity, is not an attribute of essence, instead, needs a mediator to be attributed as such, for example man and horse which are united in being animals.[2]   
The Real One is either an entity which is ‘unity’ itself; which is the very absolute existence (wjujud-e-sirf) which does not have a second and repetition; here ‘one’ and ‘unity’ are the same, and it is called ‘true unity’ (wehdat-e-haqqah). Or, it is an entity which is not ‘unity’ itself, but is attributed with it, like ‘one’ person. It is called ‘non-true unity’ (wehdat-e-ghair-e-haqqah). The ‘one’ with an ‘non-true unity’, is either ‘specialized one’ (wahed-e-bil khusoos) or ‘generalized one’ (wahed-e-bil’umoom). The ‘specialized one’ is the numerical one, with the repetition of which numbers are formed; the ‘generalized one’ like one species or one genus. The ‘specialized one’ is either in a state that, as it is indivisible from the aspect of the attribute of unity, it is also indivisible from the aspect of the entity which is attributed and is ‘the subject of the accident’ (ma’roodh) of unity; or, it is divisible from the aspect of the attributed entity and ‘the subject of the accident’. The former condition is either the very ‘intension’ (mafhoom) of unity and indivisibility, or [something] other than that. The other is either unqualified for ‘positional indication’ (isharay-e-wadh’ee) or, is something that is qualified for it. The one unqualified is either related to and dependant on matter, for example the ‘self’ (nafs), or not, for example the incorporeal [or non-material] intellects (‘uqool-e-mujarrud). The one accepting division from the aspect of the entity and ‘the subject of accident’ is either divisible essentially (bi-dh-dhat), such as unitary quantity (miqdar-e-wahed); or accepts division by accident (bil ‘aradh) such as ‘one natural body’ which accepts division from the its quantitative aspect.
The ‘generalized one’ is either intensional (mafhoomi), or is generalized in the sense of existential vastness (sa’aye wujudi) such as the ‘extended existence’ (wujud-e-mumbaset). The intensional generalized one is either ‘one’ of the ‘species’ type (wahed-e-no’i) such as human being; ‘one’ of the ‘genus’ type (wahed-e-jinsi) such as animal; or of the ‘accidental’ type (‘ardhi) such as walking or laughing [in relation to man]. The non-real one (wahed-e-ghair-e-haqiqi), was defined as one which due to a form of union with other, was attributed with unity such as: Zaid and Amr who are united in being humans, and man and horse which are united in being animals.[3]
Here mentioning one point is necessary and that is, in the opinion of the ‘transcendent philosophy’ (hikmat-e-muta’aaliyah), the ‘true unity’ (wehdat haqqah) is also of two types:

The purpose of this classification is to differentiate between the attribution of ‘true unity’ to the Exalted Essence of Allah and its attribution to others, meaning that the ‘true and real unity’ is specific for Allah the Almighty’s Essence; and in case of all others ‘true but shadowy unity” is used.

One of the basic discussions, on which the acceptance of unity of existence depends, is the issue of ‘true unity’ (wehdat haqqah) or the same ‘unity in multiplicity’ (wehdat dar kathrat) and ‘multiplicity in unity’ (kathrat dar wehdat). That is it must be looked into that how does a thing while being one is multiple, and while being multiple is one, at the same time and how can human logic and reason accept it, and do not consider it as opposites and contradictory?
One of the best ways of explaining and accepting this issue (the union of unity in multiplicity, and multiplicity in unity), is the principle of ‘Non-Composite Reality’ (baseet-ul-haqeeqah).[5] According to Prof. Allama Hasan Zadeh Amoli, it is amongst the wonders of this principle that the extreme disagreement of one of the sides is in itself an evidence of the existence of the other. Because, extreme non-composition and unity requires becoming whole [or entire] and complete, which is extreme multiplicity. Similarly amongst the other wonders of this principle is the fact that the ‘non-composite reality’ while being everything, at the same time is not any of them. However, because its being everything is from the aspect of actuality (fi’liyat) of things, and its being none of them is from the aspect of their imperfection and limitation, it is not contradictory.[6]
The respected scholar, in another explanation while elaborating unity in multiplicity says:
“From the words of the people of spiritual discovery and research, which end in guidance, it is gathered that the ultimate perfection of all attributes depends on this that because of the occurrence of the opposite does not decline and become weak; rather harmonizes and gets settled with its opponent in the string of conciliation, and derives energy from the collection. Hence in the Quranic Verses and traditions of the Spiritual Ambassadors, on many occasions, in the ornaments decorated with the pearls of the Divine Names and Attributes, opposing concepts are seen [together], for example; “He is the First and the Last, the Manifest and the Hidden, and He has knowledge of all thing[s]”[c], “Gracious and Omnipotent”, “Beneficial and Harmful”, “Preventer and Opener”, “Exalter and Abaser”, “Guide and Misleader”, “Empowerer and Humiliator” which are among the Most Beautiful Divine Names (asmaye husnaye Ilahi). The ‘true and real unity’ is termed as the ‘collective unity’, and this plurality as the ‘luminous multiplicity’. For this multiplicity [they] have said: “Kulla ma kanat awfur, kanat fi al-wehdate awghar.”             
Therefore Hafez says:
“The disturbed hair are the cause of our union
If such is the fact, more must they be disturbed”
He also says:
_________________________________
I gathered collectiveness from those disturbed hair”
The ‘hair’ means the luminous multiplicity which is a veil for the face
“Yet lasting is the Face of your Lord, Majestic and Munificent”[d]
Abu Saeed Abul Khair says:
“Daei combed the curly hair of that crescent
Put on the face the perfumed hair
Strategically covered the beautiful face
So no stranger could recognize it.”
Aaref Shabistari also says in his The Garden of Secrets (Ghulshan-e-Raaz):
Ask not the story of the tranquilizing hair
They are the chains for the crazy affectionate[7]

It seems that the key to solving the problem of ‘unity of existence’ should be sought in the concept of ‘Manifestation and Theophany’ (tajalli wa zuhur). The concept of manifestation and theophany is one of the central discussions of mysticism (irfaan) and shows its presence and has a key role in all the mystical problems, such that with its correct comprehension it can be said that the root of mysticism has been understood and other mystical problems are like effects and results of it.[8] Its because of this that when Sadr-ul-Mata’alliheen linked back casualty (illiyat) to revelation, manifestation and theophany, declared that through this new discovery, philosophy has reached its perfection.[9]

As explained earlier, the Gnostic ontology and world view is based on the system of manifestation and theophany. To elaborate [we must say]: Based on Gnosticism, existence is confined in the Exalted Essence of God; everything other than Allah is [just] an appearance (namud), theophany (zuhur) and manifestation (tajalli) of Him. According to this explanation Allah the Exalted, who is pure existence, the intensity (shiddat) and perfection (kamaal) of His Being results in an affection of theophany and manifestation. The Essence of God Almighty is affection and love itself. And the most adored thing for the Almighty, is the observation of His own Essence through the observation of [the glorious] Self-Essential affairs [or dimensions] which is termed ‘istijla’ [revelation of the Almighty’s Essence for Himself in ‘determinations’ (ta’ayyunaat)]. And this complete ‘istijla’ (istijla-e-tamm) cannot be achieved but by His theophany in each and every one of the dimensions.[10]
The Gnostics believe that the level of the Essence is (before the manifestation of) the Unseen of the Unseens (Ghaib-ul-Ghyub) which is in absolute secrecy and latency (kumun).[11] The very first manifestation and level of determination (ta’ayyun) is the same manifestation and determination of the Essence for the Essence Itself, which is termed the ‘True and Real Unity’ (wehdat haqqah haqiqiyah). A Gnostic reaches [finds] this ‘True and Real Unity’, through the attention to the ‘Unseen Identity’ (hoviyyat-e-ghaibi), without keeping in view the meanings and conditions. Because this Unity does not differ much from the Absolute Unseen (ghaib) and Identity (hoviyyat), i.e. it lies on the borders of determination (ta’yeen) and non-determination (la ta’yeen),[12] it follows that firstly: The term ‘level’ [or rank] and ‘determination’ is not used for it, instead it is sometimes named ‘Absolute Identity’ (hoviyyat-e-mutlaqay) and; secondly: Because nothing exists other than it, and it encompasses everything, along with being present in the interior (baatin), has an exterior (zahir) theophany, at the same time. As a result has two, interior and exterior faces[e]: The inner face is called the Station of Unity (martabay-e-Ahadiyyat) which means that the Almighty’s observance of Himself as a Possessor of all the Divine Aspects [and Dimensions] and having all the Names and Attributes of Perfection, though without determination and detail, rather in a simple (baseet) state (observance of the detailed in a succinct state). However the exterior face of the ‘Real Unity’ is named the ‘Rank of Oneness’ (martabay-e-wahidiyyat) which is the very station of the theophany of the Names and Attributes, meaning the Almighty’s observance of His Essence in a state of Possessing all the Names and Attributes, in detail (observance of the succinct in a detailed state). ‘Oneness’ (wahidiyyat) is the source of multiplicities [and plurality]; in a way that the multiplicities initially come into being in a ‘cognitive’ (ilmi) condition, meaning that all the essentials of the names, attributes and quiddities (mahiyyat) of all the externally existent things achieve a ‘cognitive’ appearance, and because there is no plurality (ta’addud) or multiplicity (kathrat) of ‘the Emanating Source’ (mufidh) and the emanation (mustafidh) in the ‘cognitive’ appearance, it is said that the ‘cognitive’ appearance of things comes into being through ‘the ‘most’ Sacred Emanation’ (faidh-e-aqdas), meaning that it is extremely sacred (aqdus) of the impurities of multiplicity (shawaeb-e-katharaat). Subsequently the (quiddities and the essentials of the Names and Attributes) achieve an external and actual (‘ainy) existence. The ‘external’ and ‘actual’ appearance of the quiddities is known as ‘the sacred emanation’ (faidh-e-muqaddus). The sacred emanation is also named ‘the extensive [or distributed] emanation’ (faidh-e-munbaset), ‘the Holy Merciful breathe’ (nafas-e-Rehmani), ‘Mohammadan Spirit’ (rooh-e-Mohammadi), ‘the first creation of Allah’ (awwal-o-ma khalaqa Allah). Although the sacred emanation is ‘one’ and in accordance with the Rule of the ‘One’ (qa’edeye al-wahed), emanates from the Real One (wahed-e-haqiqi), and like Allah, who is the First (awwal) and the Last (aakher) at the same time and nothing is second to Him, the sacred emanation also does not have a second or third at all because is the theophany (zuhur) of the same “He is the First and Last, the Manifest and the Hidden”.[f] However with respect to the determination of levels [or ranks], which belong to it, is divided into three worlds: “souls” (arwah) “images” (mithal) and “bodies” (ajsaam).[13]
In any case, because, from the Gnostic viewpoint, the Essence of the Almighty is attributed with the ‘Real Unity’ (wehdat-e-haqiqi) and the ‘real absoluteness’ (itlaq-e-haqiqi) of His Essence is considered one of the established [facts] of Theoretic Gnosticism (irfaan-e-nazari), and its result is the Presence of the Almighty in all the multiplicities, this presence is not horizontal (ardhi) [i.e. the Presence would exist alongside the multiplicity], rather it is an Existential Presence (hudhur-e-wujudi) which is the multiplicity itself. Hence the ‘Reality’ (haqq) in accordance with its absoluteness, has an Existential Presence (hudhur-e-wujudi) in the abode of multiplicities too. However the Existential Presence of the Absolute (mutlaq) with a ‘source of division-absoluteness’ (itlaaq-e-maqsami), does not indicate the invalidity of the multiplicities.
In other words, in order to analyze multiplicities, the theoretical Gnosticism puts forward the system of manifestation (nizaam-e-tajalli). In this system multiplicities are manifestations and theophanies of the ‘Reality’ (haqq). Manifestation is the departure of the Absolute from its station of absoluteness, and adoption of conditionality (taqayyud) and determination (t’ayyun). It must be mentioned that when the Absolute descends (tanazzul) from its status of absoluteness to the status of conditionality, it is still the same ‘Absolute as a source of division’ (mutlaq-e-maqsami) which has been conditioned; not that the Absolute and the conditioned come face-to-face with each other, and an extensional dichotomy (tabayun-e-misdaqi) is supposed between them. The Essence in its station of Absoluteness embraces all the determinations (ta’ayyunat). However, these multiplicities in the rank of Essence do not have a detailed picture against each other, i.e.; if the Essence includes the name of ‘Guide’ (Hadi) it is not possible to extract the name of ‘Misleader’ (Mudhill), by referring to its antagonism (dhiddiyyat) (or its dissimilarity) with the name of ‘Guide’. Rather, the Essence because of its real absoluteness includes the names of guide and misleader, along with emphasizing the point that, in the rank of the Essence, [in] which because of its absoluteness, no plurality can be supposed, the names and multiplicities are present in a sort of a combination (mundamij) and latent (kamin), and not in a detailed and separated form or opposing each other. Now, when this latency (kumun) reaches the apparency (burooz), and that combination inclines towards detail, the phenomenon of manifestation takes shape. Therefore, a reality has one level of latency (kumun), which, in a sort of combination, is united with other realities; and, has another level of apparency (burooz) in which it is situated against other names and attributes.
It goes without saying that the ‘Real Absolute’, in the very state of its permeation (sarayaan) in the multiplicities, in the station of the Essence, is above permeation. Because, as already said, in the rank of the Essence no opposite is supposed, such that permeation or non-permeation be discussed. But when the same Absolute Essence with a ‘source of division-absoluteness’ (itlaaq-e-maqsami) and Attributed with the property of Above-permeation (fauq-e-sarayaan), manifests in the ‘conditioned’ (muqayyadaat), its permeating identity (hoviyyat-e-sarayani) becomes ‘actual’ (bi al-fail), which is itself a name amongst the names of the Reality.[14]       

To explain the ‘unity of existence’ and the system of manifestation and theophany, many examples can be quoted; one of the best [or beneficial] of which is the human ‘self’. According to the transcendent philosophy (hikmat-e-muta’aaliyah) the unity of the self is one of the ‘true but shadowy unity’ type. The connection of the ‘self’ with the faculties is an illumination (isharaqi) connection and relation (rabitay wa idhafay); the faculties are various aspects and manifestations of one ‘self’. This unity is not only non-contradictory with plurality, rather the stronger the unity, the more multiplicity will it embrace. Because the stronger the existence, as its unity and simplicity becomes stronger, its collectivity also becomes more perfect and, becomes united with more multiplicities. In other words, in the manner in which the self’s movement towards perfection results in its unity with the faculties, and similarly it is also followed by its universality, [] the higher the levels it achieves the more increase it achieves its unity and universality grows too.[15]
The Philosopher Sabzwari in his commentary on Asfar, says: The unity of ‘self’ and faculties does not mean that the self is a collection of faculties, because a collection, does not have a real existence and unity. Similarly it does not mean unity of the self and faculties, because unity (in the sense that two things without losing their individualism and identity transform into one thing) is rationally impossible. In the same way, abandonment (tajafi) of one state (maqaam) and adoption (talabbus) of another is also not intended; instead it means that the self is the one constant secured in all the levels [and ranks]. When it is attributed with its lower levels, it does not lose its superior station; [and] when it is decorated with ethics of the higher levels, does not lose the properties of the lower levels. And what is meant is that it has the ‘true but shadowy unity’ (wehdat-e-haqqay-e-zilliyeh), in which the abstraction of various concepts from its horizontally and vertically placed levels (marateb-e-tooli wa ardhi) is not contradictory with its unity.[16]                
Sadr-ul-Muta’allihin, while explaining the unity of the self and faculties, says: On the one hand we have a conscientious knowledge (ilm-e-wijdani) that our self and essence are one; we know that the very thing in us which recognizes universalities (kulliyaat), also recognizes the particularities (juz’iyyat); the very thing which creates love and lust, gets angry [and annoyed], and ….
On the other hand, one cannot say that the unity of self and faculties is that of a synthetic relationship (nisbat-e-ta’lifi) type, such as the relationship between a commander and [his] army, or between a [father] and his children. Rather it must be said that the self and faculties have a natural unity (wehdat-e-tabee’ee) which has different aspects.[17]
The Philosopher, Agha Ali Mudarris Zanwari [or Zauri], writer of the book ‘Bada’y-ul-Hikum, in his commentary on Asfaar, explains the ‘unity of the ‘self’’ in a very interesting and complete manner. He says: The unity of the self, which is its existence itself, is a specific kind of unity which is collective unity (wehdat-e-jam’ee) which unites dimensions of existence which is ‘unity in plurality itself and plurality in unity itself’. Self, with reference to its very Essence and the last condition has a single existence (wujud-e-wehdani), and with reference to its consequents and dependents of its essence, which is the same essence in a condition of permeation (sarayan) and descending (nozool), has a scattered (farqi), divided and plural existence. Hence self in its essential (nafsaani) existence does not have a condition of collection and separation, and is free of these two conditions, is also collective and also divided. For example the source of its existence is a level, from various levels of it, and antecedent in time (taqaddum-e-zamani) but holds posteriority in from the aspect of essence (muta’akhur az nazar-e-zat). The levels antecedent in time, from the aspect of the essence are the very levels which are posterior in terms of essence, in spite of not being the essence itself from the aspect of limitations and the ascending and descending levels. Hence the self in its ascendency is descendant, and in its descent is ascendant; in its impurity (with matter) is clean, and in its cleanliness is contaminated (with matter); in its attachment (being material) is immaterial (mujarrad) and in its immateriality, is attached (is material). It is drowned in its branches and faculties, but not like drowning of something in the other, and is out of them, but not like a thing which is out of the other. Anyone who recognized himself knows his Lord. These are the very monotheistic secrets (asraar-e-tawhidi) that were termed by the Imams AS, sometimes as ‘a thing between the two extremes’ (amrun bain-al-arayn) and sometimes as ‘a state between similitude (tashbeeh) and purification’ (manzilatun bayn tashbeeh wa tanzeeh).[18]
The Mystic Shabistari, regarding the unity and multiplicity of man, says:
“The universe has become man, and man universal
There couldn’t have been a better explanation
You are the plural, which is unity itself
You are the ‘one’ which is plurality itself
You are the centre of the universe
Understand yourself, ‘cause you are the hidden world
From everything in this universe, inferior or superior
An example can be found in your body and soul
The universe is like you, a specific individual
You have made it your soul, and it has made you its body”

                         
           
     

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