In the early '90s, punk rockers began removing the trademark elements of their music -- speed, anger and ugliness -- to find out what it was about their music that really mattered, in the process creating the adventurous and introspective genre now known as post-rock. Right now, the exact same thing is happening in heavy metal. The nihilism, violence and fantastic speed that have made the past couple of decades so much fun for minions of the dark lord are turning up missing; have you heard that Sunn O))) is even playing shows without guitars? Blasphemy. L.A.'s Isis doesn't take things quite so far, instead merely trying to turn metal's hellish blast down to a slow burn. This project began with 2002's Oceanic and continued in 2004's Panopticon, and now in the confoundingly restrained In the Absence of Truth. The latter strikes a weird balance between chiming post-rock and crushing riffs, in which neither can really be said to dominate. Scorching, intermittent flares of genuine rage lend context to Truth's twinkling guitars and glassy, wandering compositions. They mark the band's metal roots, but Isis is quickly leaving heavy metal behind in favor of something solemn, intellectual, contemplative, something almost meditative. Like D. Boon's maxim, "Punk was whatever we made it to be," the record's title expresses the band's view of the possibilities of post-metal. The full quotation, attributed to 12th-century Persian mystic Hassan-i Sabbah and adopted by Beat icon William S. Burroughs, reads, "Nothing is true; everything is permitted."