PT16.S3.Q12 - Retina Scanners

Clemens_

PT16.S3.12 – Retina Scanners

This argument deals with retina scanners, machines that scan the blood vessel patterns in people’s eyes and stores these patterns, such that the scanners can recognized previously scanned patterns. The author furthermore posits that no two eyes have identical blood pattern vessels in their retinas, which seems to suggest that any given person has at least two such patterns, one for the left eye and one for the right one. The author then infers the conclusion that “[a] retina scanner can therefore be used successfully to determine for any person whether it has ever scanned a retina of that person before.”

We are supposed to identify a necessary assumption for this argument, i.e. an assumption that must be true for the conclusion to follow from the premises. Under timed conditions, I chose (B), which posits that everyone’s left and right eyes have identical patterns. I took this to be necessary for the conclusion to follow, due to conclusion’s scope (the conclusion is about “for any PERSON who ever had a retina scanned,” not about “for any given RETINA that ever has been scanned”). However, (B) seems to be false, for at least two reasons: (1) (B) goes against the information we get in the stimulus, where we are explicitly told that no two retinas have identical patterns. (2) (B) does not seem necessary for the rest of the claim that the conclusion seeks to establish (“Retina scanners allow you to answer the question, has one of the this person’s retinas ever been scanned?”). To make (B) a necessary condition, the conclusion would have to say something like “Even if you only scanned one of this person’s two retinas beforehand but not the other, retina scanners allow you to determine whether either of this person’s retinas has ever been scanned before.” However, (B) is not necessary for the way the conclusion is actually stated; the conclusion never says that the evidence to consider for any given person is a scan of only one of their retinas, as opposed to two.

The right answer choice (A) avoids this mistake by blocking another possible objection: What if people’s retina patterns change over time? Wouldn’t this make it impossible to recognize past scans later on, contrary to the argument’s conclusion suggests? (A) blocks this possible objection by establishing: Even if people get e.g. eye sicknesses, the patterns in their retinas remain unchanged over time.