Impulse

The instant-speed dig that taught blue what selection should cost. Looking four deep at instant speed and keeping one is generous on the surface, but the design discipline lives in where the other three go: the bottom of your library, not the top, so you cannot stack your next draws or feed a deck that wants to manipulate the top of its library. You pay two mana to convert one card into the most useful of four, and the cost of that conversion is burying the three you passed at the bottom of your library. That tradeoff (refinement now, depth later) became the template every blue cantrip-with-selection has negotiated against since. Compare the cheaper, shallower hits like Opt and Anticipate, or the sorcery-speed versions that look deeper but give up the stack window: the rate here, two mana for four cards seen at instant speed with no card disadvantage built in, has held up because the bottoming clause is doing the balancing rather than the mana cost. The payoff scales with a clear plan, since the deeper you know your own deck the more reliably one of four is exactly the card you needed. The instant timing is the load-bearing detail: it lets you hold up the dig as a bluff, fire it on the opponent's end step to find a land or an answer, and keep your mana untapped until you know what the turn demands.