We have, to this point, treated all attention paid as more or less the same, which is a reasonable way to broach a complex subject. And it is all the same, insofar as everything that captures it enters our mind the same way. But of course there are differences in quality of attention; watching an airplane fly overhead is not as involving as being engrossed in a film. We might half listen to a professor babble, but our ears prick up at the sound of a marriage proposal. The most basic dividing line is likely between transitory and sustained attention, the former quick, superficial, and often involuntarily provoked; the latter, deep, long-lasting, and voluntary. What matters for present purposes is that selling us things relies mainly on the former—on which the attention merchant thrives—but our happiness depends on balancing the two.

Tim Wu, The Attention Merchants, loc. 439