We use the word thing here to refer to a gathering of law givers - that is legal persons - that are assembled is such a way as to be capable of acting - that is taking decisions.
An __object__ or __thing__ is an inanimate material object as distinct from a living sentient being. > I'm not a thing, not a work of art to be cherished
But the origin of the word is entirely different. it comes from the earliest form of jury or gathering, where the thing (assembly) was the first court and democratic gathering. The thing is important - the crux of the matter.
It is the here that the idea of solidity or reality emerged. First there was only dreams, visions and spirits. Everything was immaterial, and individual. Nothing was certain or solid. We would and could argue over everything, until we could refer to something solid. The thing is...
The placing of number on an item - that is of a price was first created by a court. The earliest coursts were the first to put a price on things such as the loss of a limb. For the sake of justice, such a price needed to be constant that is it was abstracted from a range of common law cases. These first decision came to represent that final argument in the settling of a feud or debt.
So solidity, and the very concept of reality started life in an important sense as a legal construct. A thing is an assembly of important people, who over time came to lay down a set of rules that made the immaterial solid. __The thing is...__ when said loudly and passionately is a call to this higher level authority.