"In primal Islam,

the word 'aql thus had a supple, comprehensive meaning. In a hadith, the Holy Prophet provides a principle that later underlay juridical definitions of human accountability (taklif): ‘The Pen does not record the works of three people: one sleeping until he awakes, the one who is mentally unsound until he regains his sanity (hatta ya'qil), and the child before maturity.’ [28] In a similar hadith we read: ‘Four [types shall be excused] on the Day of Resurrection: a deaf man who could hear nothing, a stupid person [ahmaq], a senile man, and someone who died in the period [fatra] between the decline of one religion and the arrival of the next.’ [29] Here the prophetic voice explains that consciousness is what defines our status as human beings. 'Aql is what makes us human, and distinguishes us from other orders of creation for which there will be no judgement.

-from pg. 8-9 of Shaykh Abdal-Hakim Murad's "Reason as Balance: the evolution of 'aql" (a paper for the Cambridge Muslim College)