A person's cognitive ability can be defined as their global ability to function, which is: everything the person says and does, 24 hours a day, 7 days a week.
A cognitive disability is caused by a reduction in the physiological functions of the whole brain which reduces the cognitive ability of the person. The reductions in the qualities of information that arouse and sustain attention and in information processing speed have a global effect on the person's cognitive ability to function and their pattern of activity performance. Everything that the person says or does is reduced.