The role of doubt in Zen Buddhist practice? The Zen monk Hakuin said that 3 things are essential to practice


  great faith, great doubt, and great resolve


I can see how faith and resolve would be essential but I don't see why doubt would be? Also wouldn't faith and doubt contradict each other unless they occur separately at different times.
This doubt is a byproduct of really paying attention to details. As you pick up bits and pieces here and there, some of them don't fit together. So you put them aside and keep going. Then, later, some of them fit together, but others still don't. You have great faith in three jewels, and great resolve to walk the path. At the same time you have no choice but to suspend your judgement on some things that don't make sense. The further you go, the more this happens. But only if you really pay attention, and only if you don't get discouraged by logical inconsistencies, and only if you are determined to follow through. Somewhere along the way you get unwavering certainty on some points, even if they contradict other, equally authoritative points. Because you are certain about these things, you can give up some other things you would never have dreamt of giving up, even though your heart sinks as you do it. You keep doing it in the name of "no hope, no fear" until you lose all ground and fall through space. This is the great doubt, the flip side of your faith and determination. It's not the goal, it's a thing you have to go through. It's not something that can or should be induced artificially though. As Master Bankei said:


  What’s worse, they [the false teachers] tell practitioners that unless they can raise a “great ball of doubt” and then break through it, there can’t be any progress in Zen.  Instead of teaching them to live by the unborn Buddha-mind, they start by forcing them to raise this ball of doubt any way they can.  People who don’t have a doubt are now saddled with one.  They’ve turned their Buddha-minds into “balls of doubt.”  It’s absolutely wrong.


So doubt must be natural to be real. That's why even though Master Hakuin is right in listing it as one of three essential things, I now say it is secondary to paying attention properly, taking note of facts and how they fit together or not.