Redemption is conditioned on the inability to make up for the fast due to a permanent incapacity, such that the ability to fast is not hoped for throughout one's lifetime. Therefore, it is only obligatory for the elderly. There is no redemption for the sick, the traveler, or the pregnant and nursing women, as well as anyone who breaks their fast for a valid excuse that allows for the hope of regaining the ability; because the condition of ongoing incapacity is absent. This is because redemption is a substitute for making up the fast, and the ability to fulfill the original obligation prevents resorting to the substitute, just as in other substitutes with their original obligations. For this reason, we say that if the elderly person redeems their fast and then becomes able to fast, the redemption is invalid.