The existing system of the world is the most wise and judicious system. It is not only based on knowledge, consciousness and will but is also the most healthy and the best possible system. No better system is possible. The existing world is the most perfect because everything and every phenomenon has its particular befitting place. All existing things stand in an inexorable relation of precedence and posteriority and cause and effect to each other. No existing thing can move from its appointed position, nor can it occupy the place of something else. Existing things have varied grades of existence, and vastly differ from each other from the point of view of defectiveness and perfection and strength and weakness. This variation is an inherent part of the grading of existence.
The best system requires the existence of the relation of precedence and posteriority among existents. The disparity is not created but it is part and parcel of the creatures. God, the Exalted, creates them based on existential capacity and merit because every created being could have accepted only that position in which it has been created.
The answer to the above question is given in four parts:
First: There is no doubt that this universe has a special order and its order is an inherent part of it. All parts of it stand in an inexorable and real relation to each other like the grades existing in the numbers. We see that number (one) stands before number (two) and number (two) stands before number (three) and after number (one). As well, every number other than number one stands after a number or before it. Every number occupies a grade and fits it and it has its rules and effects. The whole numbers, which are not limited, form an order which is inherent to them. The same inherent order also pervades the creatures in the universe. That is to say, God did not create things per chance or without establishing relations between them; He did not put them one after another to make a line of incongruous things. In fact, He created them with an order.
Second: The existing system of the world is the best system imaginable. It is not only based on knowledge, consciousness and will but is also the most healthy and the best possible system. No better system is possible. The Holy Quran says: “He Who has made everything which He has created most good.”
Third: A series of general and universal laws pervade the universe according to which God, the Exalted, runs and manages the world. He never does an action that violates those laws and there is also no change in the rules governing the universe. The Quran says: "No change will you find in Allah's Sunnah (way of dealing)."
Fourth: One of the laws existing in this universe is the law of cause and effect. According to it, every existent in the system of the world has a fixed position and place. The absence of the law of cause and effect implies that every existent can be the cause of anything. Thus, it is possible in the said case that every effect may replace a cause and every cause may replace an effect. For example, the effect of a match flame might be the same as the flame of the sun whereas it is not the case because if it were so, there would be chaos and anarchy everywhere.
The pervasiveness of the law of cause and effect in the world is very clear and tangible because every effect has its own special cause and every cause has its own special effect. The existing grades among the creatures are not contractual or mentally posited like those existing in our social relations. In our social relations, we see a person acting one as a low-ranking office in a department; the next day or month he may serve as the head of that department. This is not the case with the existents in the concrete external world. The relations and grades are real and actual. Based on the pervasive law in the universe, a goat cannot be human being nor can a human being be a goat. The disparity of the existents is inherent to them.
As was said earlier, every creature is created on the basis of its existential merit. That is to say, God creates things based on their intrinsic capacity and potentiality. The grades and positions determined for the existents based on their natural capacities are not like social posts and grades (boss and subordinate) to be changeable. In fact, these positions are like distinct geometric shapes being inherent to the existents. When we see that the property of an equilateral triangle is that its total angels are equal to two distinct angles and the property of an equilateral square is that its total angels are equal to four distinct angles, this does not mean that they have been given one of the properties of two equal angles and the other, the property of four equal angles. Hence, we cannot say why they have done injustice to triangle by not giving it the property of the four equal angles or vice-versa. That is because the equiangular triangle cannot have but that fixed property and fixed capacity.
When it comes to existing grades among the creatures, the same law applies to them. The fact that solid things do not grow and having feeling but plants grow and do not have feeling and animals grow and have feeling is inherent to each one of them. This variation is an essential part of the grading of existence. It is no discrimination and cannot be considered to be contrary to justice or wisdom. Discrimination presupposes the existence of two beings of equal potentialities on one of which a favor is bestowed and from the other it is withheld. But the truth is that where the disparity is due to any inherent deficiency, the question of discrimination does not arise. It is true to say that every thing in the world accepts only a special form or mode of existence and God also grants it that form of existence. Bu Ali Sina (Avicenna) says: “God did not make apricot as apricot but he created it (as apricot from the beginning.)
In order that the point be made further clear, you should go through the following example: A car is made up of many different parts such as engine, body, wheels, steering, light, gear, screws etc. The manufacturer did not manufacture them in equal parts in the beginning; rather he manufactured each engine, body, wheel, steering, light, gear, and the rest separately and fitted each of them in its appropriate place so that the car could function properly.
 - Sajda, 7.
 - «ولن تجدَ لسُنةِ الله تبديلاً». Al-Ahzab, 62.
 - Mutahhari, Murteza, Divine Justice, pg. 102 – 107, Sadra, edition 30, 1378 (1999).
 - «وما جعل اللهُ المشمشة * مشمشةً بل أوجدها».