But now, how does knowledge come into the middle of the things?
In order to be deemed physically real, a thing must show a certain constancy in its appearances. This is a criterion that, though naturally used, did never become part of physical theory. Because it is much too fundamental. It is rather logical. That is where it belongs, in the sphere of logic, of mental things, of knowing.
Actually, we have made out this steadiness as being essential for knowledge; indeed, we have used it to define knowledge. Which, of course, makes sense only if complemented by the counterpart, the change, that defines activity.
Not only is knowledge the firm ground we can rely on, resting in sameness like a rock in the rough sea of change and activity; it also manifests itself in characteristic activity. It is a bridged and grasped difference. It is captured activity, calmed down, but always potentially present.
And that is exactly how we have to imagine a material thing, as full of and driven by activity. An activity, however, that is somehow tamed, manifesting itself in continual appearances of the thing. But then, on closer examination, these appearances turn out to be interactions with other things. In the context of physics they may determine the things’ mass and energy, for example, two terms that correspond, in the main, to knowledge and activity, although the latter are meant to be much more general, principal, marking a logical conception.