The first sign that a distractible child is doing “better” — with age or Adderall, say — is that he sits still. This is why the A.D.H.D. diagnosis, which popularized the idea of an “attention span” that can be pathologically short, grew out of the old “hyperactive” diagnosis.

So how did we find ourselves with this unhappy attention-span conceit, and with the companion idea that a big attention span is humankind’s best moral and aesthetic asset? In other eras, distractibility wasn’t considered shameful. It was regularly praised, in fact — as autonomy, exuberance and versatility. To be brooding, morbid, obsessive or easily mesmerized was thought much worse than being distractible. In “Moby-Dick,” Starbuck tries to distract Ahab from his monomania with evocations of family life in Nantucket. Under the spell of “a cruel, remorseless emperor” — his own single-mindedness — Ahab stays his fatal course. Ahab’s doom comes from his undistractibility.

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?

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.”

the digital-age equivalent of souls