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Why is it difficult to draw a sharp line between observable and unobservable things?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — Philosophy of science Aristotle, marble portrait bust, Roman copy (2nd century bce) of a Greek original (c. 325 bce); in the Museo Nazionale Romano, Rome.

In the philosophy of science, the difficulty in drawing a sharp line between observable and unobservable entities stems primarily from the lack of a clear, non-arbitrary threshold for what constitutes an "observation".1

The American philosopher Grover Maxwell argued that the concept of the observable is inherently variable, depending on the range of available technological devices. Many people rely on aids such as eyeglasses or contact lenses to perceive the world, while others utilize more advanced tools like magnifying glasses, microscopes, and telescopes to extend their sensory reach. Because there is a continuous spectrum of such devices, Maxwell noted that it is logically difficult to determine exactly where the "decisive ontological shift" occurs—the point at which an entity transitions from being observable to unobservable.1

Furthermore, the distinction is complicated by the view that there is no "neutral observation language". If all scientific language is laden with theoretical presuppositions, then there is no objective basis for treating statements about unobservables differently from those about observables. This challenges the traditional attempt to privilege observation as a foundation for scientific truth.12