We seem to know a great deal about attention spans, those constituents of character that have become the digital-age equivalent of souls.

Everyone has an attention span. It can be short or long. Long is good. Good scholars, good citizens and good children have long attention spans. Attention spans used to be robust; now they are stunted. Technology — MTV, the Internet, the iPhone — shriveled them. Nicholas Carr, who argued in “The Shallows” that Web use practically causes brain damage, told PBS that technology is “pushing even more distractions and interruptions on us” and thus will never “return to us our attention span.”

At the same time, there is a pro-technology view of attention spans — rarer, but no less confident. Science writers like Jonah Lehrer have pointed to studies that seem to demonstrate perfectly respectable attention spans in gamers and Web users.

And so polemicists of various stripes continue to calibrate the effect of technology on attention spans. But I’m surprised that anyone ventures so far into this thicket of sophistry. I get stuck much earlier in the equation. Everyone has an attention span: really? And really again: an attention span is a freestanding entity like a boxer’s reach, existing independently of any newspaper or chess game that might engage or repel it, and which might be measured by the psychologist’s equivalent of a tailor’s tape?