 Solution

An empiricist can distinguish what could be observed by creatures like us from what is posited to account for observations. This is not a distinction between what is already known and what is unknown. A distant ordinary object can be observable in suitable circumstances even if no one has seen it; successful detection of an electron need not make an individual electron directly visible to an unaided human observer. The criterion concerns a capacity, not the history of a particular observation.

Bas van Fraassen's constructive empiricism uses this distinction to separate accepting a theory as empirically adequate from believing every claim it makes about unobservable entities. Instrumental outputs can be treated as further phenomena that the theory must accommodate, without identifying every instrumental detection with unaided perception.^[ref] On this view theoretical language remains literal; agnosticism about an unobservable entity is not the assertion that its name is meaningless.

The difficulty is to make the boundary principled. A microscope, a telescope and a particle detector extend our practical reach in different ways. Instrument use is continuous with ordinary perception in its dependence on lenses, biological processing and learned interpretation. An observer can also be mistaken about a readily visible object. Thus a distinction between direct observation and instrument-assisted inference cannot simply be a distinction between certainty and fallibility.

The empiricist need not claim that the boundary is sharp in every marginal case or fixed independently of science. Information about human perception can help identify what humans could observe. That makes the criterion scientifically informed and relative to an epistemic community; it does not by itself make it inconsistent. But someone who uses accepted science to identify human limitations must explain why such acceptance does not already commit them to all of that science's unobservable ontology.

The distinction is useful as a proposal about the aim and evidential commitment of scientific acceptance. It permits disciplined use of a model while withholding some metaphysical belief, and it makes the difference between fitting appearances and describing hidden structure explicit. Its usefulness does not establish it as the uniquely rational boundary of belief. A scientific realist can argue that independently calibrated instruments and successful interventions justify belief beyond unaided observability. The main dispute is how much belief that success warrants, rather than whether scientists should use the instruments or take their results seriously.