Change is an essence’s transitioning from being attributed with some accident, to being attributed with some other accident. When the first accident ceases to exist, the second emerges into existence.
For example: a body that changes from rest to motion, transitions from being attributed with rest, to being attributed with motion. When rest ceases to exist, motion then emerges into existence.
Allah is immutable because He is beginningless, and whatever has the potential to change is necessarily emergent. We argue for the emergence of all beings with the potential to change with following argument:
- Whatever has the potential to change, is inseparable from accidents.
- Whatever is inseparable from accidents, is emergent.
- Therefore, whatever has the potential to change, is emergent.
Premise 1: Whatever has the potential to change, is inseparable from accidents
The first premise proposes that it is impossible for a being that can change, to exist without any accidents.
This is true by definition of change. Since “change” occurs when a being loses one accident, and acquires another. Thus, change requires the changing being to be attributed with some accident. For if this being were not attributed with any accidents, then it would be impossible for it to lose an accident in order for change to occur.
Premise 2: Whatever is inseparable from accidents, is emergent
The second premise proposes that a being inseparable from accidents, is itself emergent.
This premise is true, because the set of all accidents is emergent. So since the being in question is inseparable from accidents, it could not have existed before the set of all accidents emerged into existence. Thus, this being’s existence is preceded by its non-existence (i.e. it is emergent).
The set of all accidents is emergent, because each accident is emergent, and the number of accidents that emerged into existence in the past is finite. Each accident is emergent because they can cease to exist, and whatever can cease to exist has a beginning. The number of accidents that emerged into existence in the past is finite, because Tasalsul is impossible.
Therefore, whatever has the potential to change is emergent
The two premises are true, so the conclusion necessarily follows. Therefore, whatever can change, must have emerged into existence. And since Allah ﷻ is beginningless, then He is necessarily immutable.
 If an accident could not cease to exist, then this accident wouldn’t be an accident. As that would make it impossible for the being attributed with it to exist without it.
 In summary: the existence of that which can cease to exist is possible, since such an essence accepts non-existence. Its existence is therefore contingent upon an extrinsic specifier to have brought it into existence. More on this here.
 Tasalsul: the belief that the past is comprised of an infinite number of events. Commonly referred to as an “infinite regression of past events”.
In summary: Tasalsul is impossible because the past is that sequence of events which leads up to, and then ends with the present moment. To claim that the past is infinite, is therefore tantamount to claiming that a sequence which came to an end is endless. And that is a clear contradiction. More on this here.