The answer is that if he is like one who has fainted, then fasting remains obligatory upon him, for a person who has fainted must make up the fast after regaining consciousness. But if his loss of reason reaches the point of insanity, such that his awareness is gone, then he takes the ruling of an insane person. The same applies to one who suffers intermittent insanity: on a day when he is insane, fasting is not obligatory upon him; but on a day when he is in possession of his reason, it is obligatory upon him.
The evidence for this is the Prophet’s statement, peace and blessings be upon him: “The Pen has been lifted from three: from the sleeper until he awakens, from the child until he reaches puberty, and from the insane person until he regains his reason.” (1)
His statement, “capable,” is the third condition—that is, capable of fasting—thus excluding one who is unable. Fasting is not obligatory upon one who is unable, due to the saying of Allah Most High: “And whoever is ill or on a journey, then let him fast an equal number of other days.” [al-Baqarah 2:184]
However, through examination and induction, it becomes clear that inability is of two types: temporary and permanent.
Temporary inability is that which is expected to cease. This is what is mentioned in the verse, so the person who is unable waits until his inability is removed, and then makes up the missed fasts, due to His saying, Most High: “Then let him fast an equal number of other days.”
Permanent inability is that which is not expected to cease. It is mentioned in His saying, Most High: “For those who can fast only with great difficulty, there is a compensation: feeding a poor person.” [al-Baqarah 2:184] Ibn Abbas, may Allah be pleased with him, explained this as referring to an elderly man and woman who are unable to fast; they feed one poor person for each day. (2) In reality,