Why do scientific realists question the distinction between observable and unobservable entities?

Scientific realists challenge the distinction between observable and unobservable entities by arguing that there is no principled, sharp division between the two, nor is there a "theory-neutral" language available to report observations.12

This skepticism is supported by several key arguments:

• Continuum of Observation: Philosopher Grover Maxwell noted that the range of the "observable" expands with technology. Because there is a continuum between seeing with the naked eye and using devices like microscopes or telescopes, it is difficult to identify a decisive point where an entity ceases to be "real" or observable.2
• Theory-Laden Language: Realists argue that all scientific language is "theory-laden," meaning it carries theoretical presuppositions. If no observation language is truly neutral, there is no logical basis for treating language about unobservables differently from language about observables.12
• Explanatory Success: Realists contend that the success of scientific theories—such as the ability to compute reaction energies with extreme accuracy—would be a "miracle" if the unobservable entities described by those theories did not exist. They argue that the best explanation for this success is that the theories, including their claims about unobservables, are at least approximately true.12