He has tried imagining her as a prostitute-he often plays this private mental game with various women he encounters-but he can't picture any man actually paying for her services. It would be like paying to be run over by a wagon, and would be, like that experience, a distinct threat to the health.
The protagonist struggles to view a certain woman in a demeaning light, despite his attempts to frame her as a prostitute, a common mental exercise he engages in with women he meets. This struggle reflects an internal conflict where he cannot reconcile her image with that of a sex worker.
He finds the idea absurd, comparing it to willingly seeking out harm, like being run over by a wagon, emphasizing his perception of her as someone not only unworthy of such treatment, but also as a figure who poses an inherent danger. The metaphor illustrates his deep respect for her, alongside his fear of the consequences of commodifying her existence.
