An "invisible" sculpture by an Italian artist was reportedly sold for $18,000 (£12,959) at an auction earlier this month.
Salvatore Garau's sculpture "Io sono" (Italian for "I am"), according to Italy24News, must be shown in a secluded room free from any interference.
It must also be housed in a space that is approximately 5 feet long and 5 feet wide. Since no one can observe the work, lighting and environmental control are optional.
Garau has been criticised for claiming that the sculpture is "immaterial," implying that it either doesn't exist or just exists in the mind of its maker, reports The Indepenedent.
The 67-year-old, on the other hand, has justified himself by arguing that the artwork is more akin to a "vacuum."
"The vacuum is nothing more than a space full of energy, and even if we empty it and there is nothing left, according to the Heisenberg uncertainty principle, that 'nothing' has a weight. Therefore, it has energy that is condensed and transformed into particles, that is, into us," he said.
"After all, don't we give shape to a God we have never seen?" he asked.
Last week, Garau displayed another invisible sculpture titled "Buddha in Contemplation" in the Piazza della Scala in Milan.
The video states: "Now it exists and will remain in this space forever. You do not see it but it exists. It is made of air and spirit."