there are enough retina scanners to store information about every person's retinas the number of blood vessels in the human retina is invariant, although the patterns they form differ from person to person there is no person whose retinas have been scanned by two or more different retina scanners diseases of the human eye do not alter the pattern of blood vessels in the retina in ways that would make the pattern unrecognizable to the retina scanner
The retina scanner, a machine that scans the web of tiny blood vessels in the retina, stores information about the pattern formed by the blood vessels. This information allows it to recognize any pattern it has previously scanned. No two eyes have identical patterns of blood vessels in the retina. A retina scanner can ttherefore be used successfully to determine for any person whether it has ever scanned a retina of that person before.
The reasoning in the argument depends upon assuming that