Scientists have discovered a spinal fluid test that can predict with 100 percent accuracy whether people who already have memory loss are going to develop full-fledged Alzheimer’s disease. They apparently don’t know whether this test works for people with no memory problems yet, but reading between the lines of the report in the New York Times August 10, it sounds as if they believe it will. And as of now, there is almost nothing they can do to mitigate, let alone prevent, Alzheimer’s.

This is truly the apple of knowledge: a test that can be given to physically and mentally healthy people in the prime of life, which can identify with perfect accuracy which ones are slowly going to lose their mental capabilities. If your first instinct is, "We should outlaw this test" or at least "we should forbid employers from discriminating on the basis of this test," congratulations—you’re a liberal. People should be judged on the basis of their actual, current abilities, not on the basis of what their spinal fluid indicates about what may happen some day. Tests can be wrong.

Fine. And now you’re going into the hospital for brain surgery. You wouldn’t be curious to know how your surgeon’s spinal fluid scores? You would be reassured if told, "Well, he’s going to have Alzheimer's, but not so you’d notice for a few years"? I doubt it.

The Americans with Disabilities Act already forbids employers from using medical tests to discriminate against a job applicant. The ADA was passed with amazingly little fuss under President Bush the Elder. It incarnates the principle that people should not be discriminated against on the basis of a disability. It’s a wonderful principle, utterly unenforceable. To take this principle seriously would mean not discriminating in favor of anyone because of his or her ability, which would be no way to run a football team, or almost any other organization. The ADA only attempts to apply the principle in limited situations, and it allows discrimination against disabilities that actually prevent you from doing the job. In short, the ADA cops out on the really tough questions: What about people with no marketable talents? Why isn’t that a disability just as debilitating as, say, a missing leg? Or what about folks who seem fine now, but whose spinal fluid tells a different tale for their future?