Define the copula "is".
Any investigation must leave some of it's terms undefined. This isn't my opinion or belief. This is basic mathematics. Of course some of those terms can be defined in some contexts, but these definition are usually relative to a particular model or formal system. It's literally impossible to ever define all your terms or establish the truth of all of your assumptions. That's why we have axioms, which are statements that we assume to be true, and undefined terms, which are terms that we don't rigorously define.
In fact, most of the time in science we don't have a completely accurate definition of an object or physical process, we just know it exists. That's literally why we perform experiments and collect data. We want to be able to construct a better description of an object. We know something exists, but we don't fully understand it and can't provide an accurate definition.