The fact (or factual) dimension is about drawing a distinction that allows an observer to indicate (intend or make a selection from a surplus of possibilities) one thing while ignoring every other possibility. However, the other possibilities remain available for future selection.
According to Luhmann,
One can speak of the fact dimension in relation to all objects of meaningful intentions (in psychic systems) or themes of meaningful communication (in social systems). . . . The fact dimension is thereby constituted in that meaning divides the reference structure of what is meant into “this” and “something else.” Thus the point of departure for a factual articulation of meaning is a primary disjunction, which contrasts something as yet indeterminate with something else as yet indeterminate. (Luhmann, Social Systems, 76)
The “this and something else” is a two-sided form. The famous gestalt image of the vase face demonstrates the two-sided form. When limited to the factual dimension, the observer sees faces or a vase, not both simultaneously.
In other words, there is a conflict, which can be handled with the aid of time. That is to say, one can use the temporal dimension to see first one image and then the other, switching (or crossing) back and forth. Crossing the two-sided form of “this and something else” takes time. It’s like flipping a coin, which of course takes time. Crossing means crossing a distinction, not a horizon. A horizon cannot be reached; so, of course, it cannot be crossed.
The social dimension introduces the difference between consensus and dissension, or consent and dissent. People often speak of getting to or reaching consensus. But consensus in itself is useless. The social dimension features the possibility of crossing from consensus to dissent (or from agreement to disagreement) and back again. Agreement and disagreement are uncrossable horizons. Communication can move toward agreement or disagreement, but it can never reach one or the other in an absolute sense. It must be possible to turn back towards the opposite horizon. This is what operational closure means.
As Luhmann puts it,
A horizon is not a boundary; one cannot step across it. At some point one must turn back, and the opposite horizon indicates the direction “back.” (Social Systems, 77)
The possibility of turning back to the opposite horizon guarantees structural instability.