How many artists can make an elephant disappear right in front of your eyes -- with everything out in the open? As I went through This Progress, one of two performance pieces by Tino Sehgal that transform Frank Lloyd Wright’s emptied-out spiral into a dreamy Socratic-purgatorial journey, the museum literally fell away. I was suspended in some weird nonspace. I felt variously shook up, spaced out, turned on, told off, intimidated, ashamed, thrilled and shocked. Yes, it’s artsy artifice; yes, it sometimes feels like you’ve stepped into a Monty Python sketch. Doesn’t matter. This show is wondrous-strange, and can produce waves of uncanny self-revelation, surprise and delight.