Collins does not define the operational level of war, but he does define "operational art"

"Military plans and operations that implement military strategies at theater level. Campaigns predominate if armed combat occurs."

He defines campaigns as:

"A series of theater-level military actions that may or may not be strategically significant."

To complete this, he defines theater operations as:

"A major politico-military or geographic area (such as Western Europe or the Middle East) for which military commanders in chief prepare plans and conduct operations."

Its not like we're discussing Bigfoot or the Loch Ness Monster. These are simply theoretical constructs to focus and guide military planning and operations. The fact that they exist is indisputable. One can debate their value, but their existence is irrefutable.