… according to Sellars’s view, the distinction between theoretical objects and observable objects is methodological, rather than ontological. That is, theoretical and observable objects are not different kinds of thing. They differ only in how we come to know about them. Theoretical objects are ones of which we can only have inferential knowledge, while observable objects can also be known noninferentially. Theoretical concepts are ones we can only be entitled to apply as the conclusions of inferences, while concepts of observables also have noninferential uses. But the line between things to which we have only inferential cognitive access and things to which we also have noninferential cognitive access can shift with time, for instance as new instruments are developed. Thus when first postulated to explain perturbations in the orbit of Neptune, Pluto was a purely theoretical object; the only claims we could make about it were the conclusions of inferences. But the development of more powerful telescopes eventually made it accessible to observation, and so a subject of noninferential reports. Pluto did not then undergo an ontological change. All that changed was its relation to us.

Sellars’ Two-Ply Account