Something (Logical)
The concept of thing has to be called “logical” because it is simply indispensable. Whenever we think, perceive, measure (or whatever) something, it is about something. This something we call “thing”. And it does not matter whether this means a verbal expression, for example, or a mental image, a notion, an idea, a law, or maybe a material thing. Purely logically, that makes no difference.
However, of course this does not mean that mental and physical things are equal in all respects, or that we know much about the things “a priori”, so to speak. It is always up to the physics (among others) to find out and to decide whether something really (physically) exists or not, with experience playing an important role.
But a lot of what is such a worry to physics because it does not fit into one of the familiar physical patterns, although it is verifiable, perhaps turns out to be not half as mysterious as still considered today. For it just has to be so, purely logically.