But our capacity to ignore is limited by another fact: we are always paying attention to something. If we think of attention as a resource, or even a kind of currency, we must allow that it is always, necessarily, being “spent.” There is no saving it for later. The question is always, what shall I pay attention to? Our brains answer this question with varying degrees of volition, from “shhh—I'm reading this” to letting our minds wander in the direction of whatever might draw it in, whether in the corner of our screen or along some road we are walking. That is where the attention merchant makes his opportunity. But to succeed he must motivate us to withdraw our attention from where it is and surrender it to something else. It needn't be a thoughtful calculation.

Tim Wu, The Attention Merchants, loc. 63