.... Fanfare of trumpets ...  
.... Roll of the drums ...  
.... Very loud noise from 76 trombones ....

THE PERFECT ANTIVIRUS

Definition. I shall now give you, free of charge, an antivirus that if used
correctly, detects all past, present and future viruses, never gives a false
alarm, and has a zero cost. Sceptical? Then watch carefully ...

P1.BAT
Echo %1 is infected by a virus!!!

You'll agree, I think, that P1.BAT will detect all past, present and
future viruses. That alone meets the "mathematically impossible" task you
set me! But, I hear you thinking, aren't there rather a lot of false
alarms? Well, you didn't say you wanted a low false alarm rate.

OK, OK. I'm used to projects where the user specification changes in
the middle. Never mind. I can deal with the false alarms ...

P2.BAT
Echo %1 is NOT infected by a virus!!!

You'll agree, I think, that P2 will never, ever, tell you that you have a
virus when you don't. Of course, it has a pretty poor detection rate. I
admit that. But I can fix it. See here ...

PERFECT.BAT
Echo Is %1 a virus? (Y/N)

If the user types Y, you run P1. If the user types N, you run P2. Remember
what I promised you? An antivirus that *if used correctly*, detects all
past, present and future viruses, never gives a false alarm, and has a
zero cost.

All very amusing, but what can we learn from this?

1. If something is superb at detecting viruses, it's no use if it gives a
lot of false alarms.

2. Anything that relies on the user to make a correct decision, on
matters that he is not likely to be able to decide about, is useless.

3. You can receive something that is *exactly* what the salesman promised
to deliver, and it's nevertheless useless.