But the biggest downside to a national ID card is that once it was in place, and in order to make the best use of it for fighting terrorism, it would be irresistible that it be transformed from something that is only practically mandatory (like the drivers license and the Social Security card – in theory you could arrange your life to live without them) into something that is legally mandatory, not merely if you want to board a plane or to handle dynamite but simply because you exist. And then, once governments and government agencies across the spectrum know that everyone must have one, it is only practical that they demand you present it for every interaction with government – for your own protection of course, just to ensure we know who we’re dealing with. And how about when you are acting suspiciously in the street? In fact, why don’t we just make it mandatory that you carry it with you at all times, and show it on demand to any government official?