Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
One of the foundations of faith is that we believe in the wisdom of Allah تعالى in His creation, His decree, and His law, and that He, Glorified and Exalted is He, does not create anything without purpose, nor does He decree anything except for a wisdom. Allah تعالى says: “Allah is the Creator of all things.” [Az-Zumar: 62]
It is part of His wisdom, Glorified and Exalted is He, that He has made people different from one another in their bodies, minds, abilities, and circumstances. Thus, among them are the healthy and the sick, the rich and the poor, and those who are mentally sound and those who are not. This difference is one of the established ways of Allah in His creation. Among His wisdoms is that He tests some of His servants through others, so that the grateful may be distinguished from the ungrateful. Allah تعالى says: “Indeed, We created man from a drop of mixed fluids, testing him, and We made him hearing and seeing. Indeed, We guided him to the way, whether he be grateful or ungrateful.” [Al-Insan: 2–3] And He, Glorified is He, says: “He is the One who created death and life in order to test you as to which of you is best in deeds.” [Al-Mulk: 2]
When a person sees someone who has been tested with a disability or a deficiency in some abilities, he recognizes Allah’s blessing upon himself, so he gives thanks to Him and asks Him for well-being, and he comes to know the power and wisdom of Allah تعالى.
A servant cannot encompass all of Allah’s wisdoms in His creation and decrees. Allah تعالى says: “He is not questioned about what He does, but they will be questioned.” [Al-Anbiya: 23] Whatever of His wisdom becomes clear to us, we believe in it; and whatever remains hidden from us, we submit to His wisdom regarding it, saying: “Allah knows best and is most wise, and we have no knowledge except what You have taught us.”
And Allah تعالى knows best.