One thing which makes causal mapping a fundamentally qualitative approach is that we do not code absences.

We do not think that the world, nor the piece of the world we are studying, is essentially a grid of variables and cases (nor a cube of variables and cases and timepoints), in which each case always has a value for every variable (at every timepoint).

If some respondents say that their headaches make them nauseous, and others do not mention headaches, even if they mention nausea, we do not interpret that as meaning that they did or did not have headaches. We do not think that having headaches, or not, is a variable which must be relevant to everyone's explanations, all the time.