Kant’s ideas are still useful even in the era of quantum field theory. Human thought is implicitly limited, as is human sense input. Human physiology perceiving what exists of the mass of the field that it interacts with has limits. Everything that isn’t self (oneself) or especially human or even animate has its own nature and being-for-itself. Mass has no mind to it (except for rare thinking beings), and objects within a field that appear as objects to thoughtful beings but not perhaps to neutrinos passing through them without interactions, like the Earth, haven’t plain boundaries in time and space- they are all part of various physical fields that has clumpiness of matter in it. I guess if one considers Hilbert Space full of particle/points it is illustrative to consider how many ways one could draw lines between the points – say there are a few billion points, making diagram objects of the points in the space. Some of the points may have different qualities-with a strong force or electro-magnetic force like a boson and won’t interact with others and some interact with all-like gravity particles (if they exist). What the points are like for-themselves is an interesting point to consider-especially if the points are like the one-dimensional point-monad/atom-bubbles of Liebnitz he thought were conscious.
E = mc2 . Mass is just energy locked into decohered loops. I am not sure what pure energy really is, or if it needs anything else including dimensions, motion or time to exist. Plainly though, measuring the flow rate of physical materials or energy is at least an apparent or secondary phenomenon useful for humans. Mass is supposed to be a secondary quality of energy.