The Cathedral of Notre Dame was built to be a house of God, its soaring Gothic sanctuary meant to inspire both awe and fear.
Indeed, one of the latest photographs of the cathedral's fire-damaged interior paints a miraculous picture of survival: a pile of charred timbers leads the eye upward to a statue of the Virgin Mary holding the crucified Jesus, a golden cross over Mary's head.
Yet Notre Dame is still subject to the laws of physics - and scores of unanswered questions about the continued viability of its structural skeleton, which rises more than 108 feet from the floor of the cathedral to the top of its criss-crossing vaults.
Did the intense heat of the fire weaken the cathedral's centuries-old blocks of stone, causing what structural engineers call "thermal shock"?
Could the mortar that holds those blocks together have been loosened by the torrents of water that put out the fire?
Those were among the questions swirling as officials tallied the damage to the beloved landmark, French companies like Louis Vuitton promised millions of euros to rebuild, and discussion began on how the reconstruction should be carried out.
At the core of that conversation is Notre Dame's structural skeleton - the pioneering combination of pointed arches, ribbed vaults and flying buttresses that made Gothic sanctuaries, in the words of the architectural historian Otto von Simson, "the threshold to heaven."
Notre Dame is believed to be the first of the French Gothic cathedrals to use monumental flying buttresses that carry the building's enormous weight of stone, lead, and glass down to earth.
And like other Gothic cathedrals, Notre Dame had a double roof system - a stone vault topped by a timber roof. That separation likely helped the cathedral to survive.
In his 1995 book, "The Stone Skeleton: Structural Engineering of Masonry Architecture," University of Cambridge professor Jacques Heyman observed, "the timber roof protects both the stone vault and the church from the weather, and the stone vault protects the church from the potential fire hazard of the timber roof."
Thus, in contrast to Monday's apocalyptic tone, when a fire official said Notre Dame might be destroyed, there was an air of guarded optimism Tuesday.
French junior Interior Minister Laurent Nunez told reporters that Notre Dame's overall structure "is holding up well, but some weaknesses have been identified." He cited the cathedral's vaulted stone ceiling and the gable of its northern crossing, "which have to be secured."
Yet while the cathedral's survival seems assured, the damage assessment is just beginning, said Tim Crowe, an associate principal at the Northbrook-based firm of Wiss, Janney, Elstner Associates, which specializes in evaluating and repairing structures but is not involved in assessing Notre Dame.
Once the cathedral's floor is cleared, scaffolding will likely have to be erected for a close-up inspection of Notre Dame's stone vaults. In the meantime, drones could provide some preliminary snapshots, Crowe said.
Securing the structure's wobbly areas is likely to require erection of temporary metal frameworks that will be in place for months, if not years. "I hope not decades," said Shankar Nair, a structural engineer at the Chicago office of the firm exp.
Adding sprinklers seems like an obvious course - as long as they don't interfere with the cathedral's beauty.
Less certain is the precise course of rebuilding: Should the cathedral be restored to exactly the way it looked before the fire?
Almost surely, yes.
There's no point in asking what the Gothic master-builders would do with today's technology. Notre Dame was beloved as it was.
But will rebuilt portions of the cathedral, particularly its picturesque spire, which was built in the 19th century, convey the same authenticity as the original?
They probably can, given the abundance of documentation available. Historic preservationists often use contemporary materials to reproduce the original building's visual effects.
Restoring the cornice of Chicago's iconic Marquette Building in 2003, for example, Chicago architect Gunny Harboe used glass-fiber reinforced concrete instead of the original terra cotta. That step, which cut costs by at least two-thirds, formed a replacement that perfectly matches the proportions, color and classically inspired details of the original.
Comparable flexibility may be needed at Notre Dame, especially when it comes to the task of replacing the timber beams that supported the cathedral's steeply pitched roof.
Made of tall, sturdy oaks that likely sprouted in the 8th or 9th centuries, the beams were a virtual forest - one that was considered one of the oldest structures in Paris. That forest is likely irreplaceable.
"Those things go away. It's like redwoods. You don't get them back," said Chicago architect Phil Hamp, a specialist in historic preservation.
There are also structural reasons to stick as closely as possible to Notre Dame's original structure, even if a modern material like steel like might be stronger than wood.
"A metal frame may start to behave differently from what the wood system had done over the last many centuries," Crowe said. "It just throws additional curve balls at the assembly."