It is a substance (jawhar). According to others, it is mutehayyiz (occupying space).
substance/substances (jawhar/jawāhir) – accident/accidents (ʿaraḍ/aʿrāḍ)
If Allah had willed, contrary to the claim of some that "from the One, only one emanates," He could have created many existents at once. If this were the case, will would be deficient and qudra (power) would be lacking. For creating many things at once is possible for His Essence and not impossible. The possible is the locus to which qudra pertains. If it is established that the first existent is one, this is because Allah the Exalted has willed it so. (Tadbīrāt, 121)
The scholars have disagreed concerning this khalīfa (vicegerent) that we call the ruh (spirit). Some of them have asserted that the ruh is an indivisible and mutehayyiz (space-occupying) substance, and that, unlike the animal body, it possesses spiritual attributes sāḥib (possessor).
Another group claims that each perception belongs to its own place, but that Allah has connected their places to the body. Accordingly, their subsistence is by the subsistence of the ruh; when the ruh leaves the body, perceptions also leave the body.
Yet another group claims that the ruh has no specific place in the body, but rather, like water permeates wool, the ruh permeates the body. ʿAbd al-Malik Ḥabīb says: The ruh is a subtle form upon the shape of the body. In the body, it has two eyes, two ears, two hands, and two feet. Each of these corresponds to the same organs found in the human body.
All these groups consider it muḥāl (impossible) for these to be accidents. When those who hold this view are asked what prevents it, they say: For us, that is, the ruh being an accident, is not a remote iḥtimāl (possibility). However, as we have heard from the Prophet (peace and blessings be upon him), the ruhs are also made happy or suffer torment, because they are enduring. Yet these two qualities are not attributes of accidents. The comparison of meaning with meaning must be made. This situation is intellectually impossible for every rational person of knowledge; the sharia has not brought anything impossible.
The second view concerning the ruh is its enduring nature. This also contradicts rational proof. For if the ruh were an accident, it would be permissible for an accident to endure. They (accidents) are always renewed. If the ruh were also an accident, it would have to be renewed. Accordingly, for every living being, there would have to be as many ruhs as the times it has passed through. All of these are bāṭil (invalid). As for the one who claims that the ruh is not a substance, his proof is also about substances.