Following in the footsteps of the Greek philosophers, Ibn 'Adi concerned himself with the ultimate human end, happiness, which he found in knowledge. However, he was primarily occupied with defending the compatibility between the concept of God's unity and that of the trinity. He reasoned that a thing can be one in one respect and many in another. Therefore, there is no inconsistency in holding that God is both one and three. Ibn 'Adi can best be described as the Christian philosopher of unity, as he devoted most of his career and used all his logical skills to defend the concept of God's unity and its consistency with the concept of trinity.
Ash-Shaykh Abu Zakariyya' Yahya ibn 'Adi was a Jacobite Christian who lived in Iraq. Born in Takrit, he moved as a youth to Baghdad, one of the most important centres of learning in the tenth century. Of Syriac origin, he was Arabized like many other Syriacs at that time. He learned logic and philosophy with the well-known logicians, Abu Bishr Matta ibn Yunis and al-Farabi, and after their deaths he became the leading logician of his time. He translated Greek philosophical works from Syriac into Arabic, wrote a number of logical, philosophical and theological treatises - the most important of which are Tahdhib al-akhlaq (Refinement of Character) and Maqala fi at-tawhid (Essay on Unity) - and established the Aristotelian school at Baghdad. His students, a mixture of Muslims and Christians, included Ibn Miskawayh, Ibn al-Khammar, who wrote a treatise on the harmony between philosophy and dogma (which may have influenced Ibn Rushd in his treatment of the same subject), and Ibn Zur'a.
In Tahdhib al-akhlaq, Ibn 'Adi sets out his ethical philosophy along Greek lines (see Ethics in Islamic philosophy). It is based on his view of the human soul, which is divided into three parts or souls: the appetitive, the courageous and the rational. The first is the lowest and is shared by humans with other animals. The last is the noblest and the distinguishing mark of being human. To follow the first is to fall into ignorance and evil; to follow the last is to adhere to goodness and happiness. While all human beings have the natural capacity for reasoning, some have the skill to reason and some do not. Those who do not may acquire it by learning the rational sciences.
Ibn 'Adi's concern with God's unity, however, was his main preoccupation. He identifies the meaning of the word 'one' as it applies to God, and investigates whether God is one in all respects or one and many. Asserting that the one is that in which there is no otherness inasmuch as it is one, he classifies six things as one: the genus, the species, the relation, the continuous, the indivisible and the definition. He then offers arguments to show that God cannot be one except in the sense of definition (definition being a descriptive statement of the essence of a thing, in that it gives the essence as it is). Since every definition mirrors an essence, God must also be one in essence.
Most Greek and medieval philosophers considered God indefinable. A definition, they say, includes a genus and a difference, such as 'animality' and 'rationality' for 'humanity'. God, being simple, cannot be included in any genus because there is nothing above him. But if he has no genus, he cannot have any difference, for the difference is what differentiates a genus. Without a genus and a difference, God cannot be defined. While agreeing that a definition requires a genus and at least one difference, Ibn 'Adi believes that God is definable because he falls under the genus 'substance', for like every substance he does not reside in anything. This is interesting because it places something, the genus 'substance', above God and makes him the cause of the substance out of which he is made. This follows from Ibn 'Adi's view that the genus is caused by its members and that God is the cause of everything. The difficulty then arises that there was a time when God had no substance.
Ibn 'Adi asserts that God is one in one respect and multiple in another. As stated earlier, God is one in definition and in essence. However, a definition is a statement, and every statement has its parts. Such parts signify separate meanings. As such, these parts are separate definitions. Thus there is no contradiction between saying that God is one in definition and multiple in definition. The multiple definitions of God signify the three attributes, goodness, power and wisdom, referred to in Ibn 'Adi's later works as intellect, act of intellection and object of intellection, respectively. Goodness signifies the Father; power signifies the Holy Spirit; and wisdom signifies Christ. The same thing may be reflected in two mirrors differently, in one as a cause and in the other as an effect. Similarly, God and the Trinity are one in essence but different as individuals. Observation of God's creatures demonstrates these attributes.
The third century ah (ninth century ad) Muslim philosopher al-Kindi wrote a treatise attempting to refute the Christian concept of the Trinity purely on the basis of logical reasoning. This treatise is, as far as we know, preserved only in the writings of Ibn 'Adi. Al-Kindi asserts that, according to Christianity, the three figures of the Trinity share substance or essence, but each is an individual by virtue of a specific property. The substance plus this property makes every one of them a composite. However, every composite is caused, and whatever is caused cannot be eternal. Ibn 'Adi responds that it is true that the three figures are composite individuals by virtue of their substance and properties, but their being composite does not make them caused, because it is possible for a thing to be composite eternally if the parts were not separate before the composition.
See also: Ethics in Islamic philosophy; God, concepts of; Logic in Islamic philosophy; TrinitySHAMS C. INATI