So, if you grant my point does this matter, defined in our arbitrary way, exist?

That depends, which arbitrary definition would you like to use?

Oh good! If the answer depends on the definition, please choose some of the more interesting examples of definitions and show how the answer changes.

Here is a defintion, if you need to get started

material substance that occupies space, has mass, and is composed predominantly of atoms consisting of protons, neutrons, and electrons, that constitutes the observable universe, and that is interconvertible with energy

Sorry, I missed that. What are you defining?

Whatever I feel like defining. It's my definition so it means what I like.

No problem. I just kind of missed it and was hoping you would repeat it in different words so I might understand it.