Islamic philosophers are divisible into two groups: illuminationists and peripateticists. Foremost among the illuminationist philosophers of Islam is the sixth century scholar Shaykh Shihab ad-Din Suhravardi (otherwise known as Shaykh-i Ishraq, but whom I shall refer to as Suhravardi), and foremost among the peripatetic philosophers of Islam is Shaykh ar-Ra'is Abu Ali ibn Sina (Avicenna).

The illuminationists are considered to be followers of Plato and the peripatetics, of Aristotle. The principal and essential difference between the two methods is that the illuminationists consider deduction and rational thought insufficient for study of philosophical questions, especially of divine wisdom (hikmar-i ilahi), and the path of the heart, asceticism, and purification of the soul as incumbent if one is to realize inner realities. Peripatetics rely solely on deduction.

The word ishraq, meaning illumination, aptly conveys a sense of the illuminationist method, but the word mashsha' or peripatetic, which means ambulant or much ambulant, is purely arbitrary and conveys nothing of the peripatetic method. Aristotle and his followers were called the mashsha 'in, the peripatetics, because Aristotle held forth while taking walks. "Deductionist" actually describes the peripatetics' method. Thus, it is more accurate to label the two kinds of philosophers illuminationists and deductionists, although I shall continue to use the more common term, peripatetic.

The major questions over which illuminationists and peripatetics differ in Islam today generally pertain to Islam and not to Plato or Aristotle. They include the questions of essentialism (isalat-I-mahiya) versus existentialism (isalat-i vujud), the unity versus the multiplicity of being, the question of fabrication (ja’l), the question of whether a body is compounded of matter and form, the question of ideas (muthul) and archetypes (arbab-i anva'), and the question of the principle of the more noble possibility (imkan-i ashraf).

Did Plato and Aristotle actually have two different methods? Did such a difference in outlook exist between the master, Plato, and the pupil, Aristotle? Was Suhravardi's method, propounded in the Islamic era, actually Plato's method? Did Plato follow the way of the Heart, asceticism and the discipline of the soul, or the illumination and witness of the heart? Was he an exponent of what Suhravardi later called experiential wisdom (hikmat-i dhawqi)?

Do the questions that illuminationists and peripatetics have been known to differ over since the time of Suhravardi (questions of essence and existence, of fabrication, of the compoundedness or simplicity of the body, of the formula of the more noble possibility, and of the unity or multiplicity of being) actually date back to differences of opinion between Plato and Aristotle? Or are the questions, at least some of them, later developments unknown to Plato or Aristotle? There were certainly differences of opinion between the two; Aristotle refuted many of Plato's theories and countered them with different ones.

In the Alexandrian period, which was the watershed between the Hellenic and Islamic eras, the followers of Plato and Aristotle formed two opposed ranks. Farabi, in Al-Jam' Bayn Ra'yay alHakimayn (The reconciliation of the views of the two sages), discusses the questions over which the two philosophers disagree and strives to resolve these disagreements. There are three basic questions on which Plato and Aristotle differed, questions different from those discussed during the Islamic era.

It is highly doubtful that Plato advocated a spiritual way, with asceticism and discipline of the soul, and witness of the heart. Thus, the notion that Plato and Aristotle bad two distinct methods, the illuminationist and the peripatetic, becomes highly debatable. It is by no means clear that Plato was recognised as an illuminationist, an exponent of inner illumination, in his own time or any time soon thereafter. It is not even clear that the term peripatetic was applied exclusively to Aristotle and his followers in his own time. As Shahristani says: "Now the strict peripatetics then are the members of the Lyceum. Plato, honoured for his wisdom, always taught them while taking walks. Aristotle followed his example, and accordingly he [apparently Aristotle and his followers were called peripatetics." Aristotle and his followers surely were called peripatetics, and this usage was simply continued in Islamic times. However, it is doubtful and even deniable that Plato was called an illuminationist.

Prior to Suhravardi, we never find any of the philosophers, such as Farabi and Avicenna, or any of the historians of philosophy, such as Shahristani, speaking of Plato as a sage advocating experiential or illuminationist wisdom. It was Suhravardi who gave this term currency, and it was he who, in his Hikmar al-Ishraq (Wisdom of Illumination), called a party among the ancient sages, including Pythagoras and Plato, exponents of experiential and illuminationist wisdom and who called Plato chief of the illuminationists.

I believe Suhravardi adopted the illuminationist method under the influence of the ‘urafa 'and the Sufis; the admixture of illumination and deduction is his own invention. But he-perhaps in order to improve acceptance of his theory-spoke of a party among the ancient philosophers as having this same method. Suhravardi offers no sort of documentation on this subject, just as he offers none on the matter of the ancient Iranian sages. Certainly, if he possessed such documentation, he would have presented it and so avoided leaving an idea to which he was so devoted in ambiguity and doubt.

Some writers on the history of philosophy, in writing on Plato's beliefs and ideas, have not mentioned his supposed illuminationist method. Shahristani's Al-Milal wa'n-Nihal, Dr. Human's Tarikh-i Falsafa, Will Durant's History of Philosophy, and Muhammad Ali Furug hi's Say r-i Hikmat dar Urupa do not mention such a method in the sense Suhravardi intends. Furughi mentions Platonic love, which is a love of the beautiful that in Plato's belief-at least as expressed in the Symposium-is rooted in divinity. It bears no relation to what Suhravardi has said about the purification of the psyche and the Gnostic way to God. Plato is said to hold: "Before coming to the world, the spirit beheld absolute beauty; when in this world it sees outward beauty, it remembers absolute beauty and feels pain at its exile. Physical love, like formal beauty, is metaphysical. But true love is something else; it is the basis for illuminate perception and realisation of eternal life."

In his History of Western Philosophy, Bertrand Russell repeatedly mentions the admixture of ratiocination and illumination in the philosophy of Plato. However, he offers no documentation or quotations that would shed light on the question of whether Plato's illumination arises from discipline and purification of the soul or is just that experience born of love for the beautiful’. Further investigation of this question must include direct study of Plato's entire corpus.

Pythagoras may have employed the illuminationist method, apparently under the inspiration of Oriental teachings. Russell, who regards Plato's method as illuminationistic, maintains that Plato came under the influence of Pythagoras in this regard.'

Whether or not we see Plato as an illuminationist in method, there are pivotal ideas among his beliefs that define his philosophy, all of which Aristotle opposed. One such concept is the theory of ideas, according to which all we witness in this world, substances and accidents alike, have their origin and reality in the other world. The individual beings of this world amount to shadows or reflections of other-worldly realities. For instance, all the human individuals who dwell in this world have a principle and reality in the other world; the real and substantive man is that man of the other world.

Plato called these realities ideas. In Islamic times, the Greek word for idea was translated as mithal (likeness, idea), and these realities were called collectively the muthul-i aflatuni (Platonic ideas). Avicenna strenuously opposed the theory of Platonic ideas, and Suhravardi just as strenuously advocated it. Among later philosophers holding to the theory of ideas are Mir Damad and Mulla Sadra. However, these two sages' definitions of idea, especially Mir Dam-ad's, differ from Plato's and even from Suhravardi's.

Mir Findiriski is another advocate of the theory of ideas from the Safavid era. He has a well-known qasida in Persian in which he propounds his own views on this theory. Here is how it begins:

Lo! The star-studded wheel, so beauteous and splendid!

what's above has a form here below correspondent.

Should this lower form scale the ladder of gnosis,

It will ever find union above with its origin.

The intelligible form that is endless, eternal,

Is compendious and single with all or without all.

No external prehension will grasp this discussion,

Be it Bu Nasr Farabi or Bu Ali Sina.

Another of Plato's pivotal theories concerns the human spirit. He believes that, prior to being attached to bodies, spirits were created and dwelt in a world above and beyond this, which is the world of ideas (or of similitudes, ‘alam-i muthul), and that they are attached to and settled in bodies subsequent to the latter's creation.

The third of Plato's theories is based on the first two and amounts to a corollary of them. It holds that knowledge comes through recollection, not through actual learning. Everything we learn in this world, although we suppose it to be something we were previously ignorant of and have learned for the first time, is in reality a recollection of those things we knew before in that, prior to being attached to the body in this world, the spirit dwelt in a higher world in which it witnessed ideas. Because the realities of all things are the ideas of those things, which the spirits perceived earlier, these spirits knew realities prior to coming to this world and finding attachment to bodies. After finding this attachment, we forgot these things.

For the spirit, the body is like a curtain hung across a mirror that prevents the transmission of light and the reflection of forms from the mirror. Through dialectics (discussion, argument, and rational method), through love, or, as Subravardi and like-minded people infer, through asceticism, discipline of the soul, and the spiritual way, the curtain is lifted, the light shines through, and the forms are revealed.

Aristotle differs with Plato on all three of these ideas. First, he denies the existence of ideal, abstract, and celestial universals; he regards the universal, or, more properly speaking, the universality of the universal, as a purely subjective phenomenon. Second, he believes that the spirit is created after the body, that is, as the creation of the body is completed and perfected.

Third, Aristotle considers the body in no way a hindrance or curtain to the spirit; on the contrary, it is the means and instrument by which the spirit acquires new learning. The spirit acquires its learning by means of these senses and bodily instruments; it had no prior existence in another world in which to have learned anything.

Plato's and Aristotle's differences of opinion over these basic questions, as well as over some less important ones, were kept alive after them. They each had their followers in the Alexandrian school. Plato's followers there became known as neo-Platonists.