Knowledge. Start with a known thing, an object.
Known thing can be in another while remaining itself. The speeding, erratic car that enters your consciousness is the one that might hit you.
If the known maintains its alterity, the knower must also maintain its own. Knowers have to affect a unity while preserving alterity.
But things that are other in place are unified only by the alterity being destroyed (when the space between them is gone); things that are other as formed and unformed are unified only when formlessness is destroyed (like when metal is cast); things that are other as determinate and indeterminate are united only as determinate (like when a body is pushed or a neuron is excited). So knowers have to be understood in opposition to this. Since this seems to rule out any sort of alteriety and unity that physics or chemistry studies, we can call all knowers non-physical.
At the lowest level, this non-physicality is affected but unknown. A world is given to a subject but it is mere presence. The world is there but it is not true. The non-physical union seems to be nothing but the fact that the animal treats the alteration of his sense organ as different from the alteration that happens when he becomes injured or satiated. The mosquito treats the scent of a victim, which alters its nose, as something different from the sort of alteration that happens in pain. The alteration of the self by an object is treated differently from the alteration of the subject as a subject.