In Organization and Decision, Luhmann argues that a system can only remember itself.
The memory function never relates to facts of the outer world . . . but only to the states of the system itself. In other words, a system can only remember itself.
For example, I do not remember a particular person; rather, I remember my own experience of the person. I can recall my experience of seeing or being with the person, and I can distinguish between that and my experience of not being with the person, including my life before meeting the person. For me, there is a before and after encountering that person, as well as a distinction between their presence and absence. These are different system states.
But when it comes to knowing oneself, there can be no before/after or presence/absence distinction. Obviously, I cannot remember a time when I was not in the world, nor can I imagine a future world without me. I cannot cross the twin horizons of my own past and future. I encounter a paradox: If there is a future without me, in order to know it I would have be there to observe it.