Why do realists question whether there is a sharp boundary between observable and unobservable things?

In the philosophy of science, scientific realists often challenge the existence of a sharp, principled boundary between observable and unobservable entities for several key reasons:

• Lack of Neutral Observation Language: Realists argue that there is no "theory-neutral" language available to report observations. Because all language is inherently "theory-laden"—meaning it carries theoretical presuppositions—there is no objective basis for treating statements about unobservables differently from those about observables.12
• The Continuum of Observation: Philosophers like Grover Maxwell have noted that the concept of the "observable" is highly variable and dependent on the range of available technology. Since human perception is frequently mediated by devices such as magnifying glasses, microscopes, and telescopes, there is an apparent continuum between direct observation and the detection of entities through complex instruments. Realists ask at what specific point in this spectrum one should draw a line to deny the reality of an entity.2
• Rejection of Privileged Status: By denying a clear division, realists argue that the theoretical claims of science should not be relegated to mere instruments for prediction. Instead, they maintain that science aims for a literally true account of reality, and that the success of scientific theories—which often rely on postulated unobservables—would be a "miracle" if those entities did not actually exist.123