“A fundamental question with respect to any conceptual framework is ‘of what sort are the basic objects of the framework?’ This question involves, on the one hand, the contrast between an object and what can be true of it in the way of properties, relations, and activities; and, on the other, a contrast between the basic objects of this framework and the various kinds of groups they can compose. The basic objects of a framework need not be things in the restricted sense of perceptible physical objects. Thus, the basic objects of current theoretical physics are notoriously imperceptible and unimaginable. Their basic-ness consists in the fact that they are not properties or groupings of anything more basic (at least until further notice). The questions, ‘are the basic objects of the framework of physical theory thing-like? and if so, to what extent?’ are meaningful ones.”

— Wilfrid Sellars, Philosophy and the Scientific Image of Man, Section II. The Manifest Image