Isis pushed the envelope so far on 2002's Oceanic and 2004's gloriously pretentious concept album Panopticon that they spawned countless imitators, which is the greatest form of flattery in some quarters.

In the Absence of Truth is the fourth full-length from Isis.

This set is not a brave leap forward -- most of us haven't caught up with the last one, and Oceanic spawned an even more experimental set of remixes in 2005 -- but a further look up the holy mountain to a new plateau, a hike to sacred ground.

Thank the gods.

On these nine tracks, Isis never nervously explore; instead they seem to know exactly what corners to look into, what crags to reach in and grab onto, what caves lead to a blinding light that holds within it both everything and nothing.

Isis is in full command this time out and as an album, In the Absence of Truth is as solidly explosive and as adventuresome as Panopticon, but their elemental control over the music is greater, therefore creating a more even production.

Aaron Turner's vocals still etch an unclean line between half-sung and guttural roar.

The lyrics are oblique and the voice is back in the mix of that nearly unrestrained savage wail of guitars, basses, drums, and keyboards.

The pace is deliberately slow and circular on all cuts.

Produced and engineered (again) by Matt Bayles, the sonic attack may be measured, but it is also pregnant with beauty and ferocity, with a guitar sound that is singular in the world of heavy metal and underground rock.

While the opener "Wrists of Kings" is fraught with thundering tom toms, a shimmering Hammond organ, and counterpoint guitars and basses, it's rhythm is the key to its melodic frame.

Turner's singing is in plain voice, but it's modal, another instrument to denote the passing of changes in the music's forms from taut, tense moments to sparse, open ones, though it's suffocating nonetheless.

The sharp contrast is found in the very next cut, "Not in Rivers, But in Drops," which once more kicks off with those huge drums while traces of Vini Reilly from the Durutti Column enter the center of the mix and form an idea that the melody of the track just moves off from.

Its gets very loud, beautifully articulated, yet reverb-drenched vocals and Jeff Caxide's bass rumble to signify something else is afoot.

A brief instrumental interlude ends in raucous, brash calamitous heavy metal that uses single frames from King Crimson's "Lark's Tongues in Aspic." The entire album could be summed up in these two tracks, gorgeously wracked though they are with violence and brutal sensuality.

But it goes from here; there's all this movement, where concentric circles ("In Root and Thorn," "Firdous E Bareen") are drawn and shattered to oblivion with a vengeance ("Dulcinea," "Holy Tears," and the closer, "Garden of Light").

It's this last track that gives an aural view of the big nothing inside the light (void) that is comprehensively full and empty, that is so relentlessly pursued on In the Absence of Truth.

But it's viewed not with a trained scholar's eye, or with critical distance, but with heavy metal's damaged eardrums on some crazy quest to unseat everything and anything in its path by reaching inside the ache of beauty and turning it inside and showing us what it really looks like, and what it took to get there.

Isis has a sound that can be copied, imitated even, but not equaled.

This is simply because the patience and discipline it takes to create a sonic world and then destroy it makes no sense to most.

Isis have been onto something from the very beginning and got to the edge of the abyss with Oceanic.

Panopticon took an oppressive yet wonderfully curious view of its surroundings.

In the Absence of Truth takes them into its dark heart squalling, whispering, crawling, drunkenly falling into its center, punching, screaming, and kicking until there is nothing left but silence.

This is rock in the 21st century, anything less is cowardice.