As Kundera brings us into the heads of Sabina and Franz to show us how experience brought to a new scenario affects the perception of the viewer, we come to an interesting philosophical question on truth:
What does it mean to live in truth? Putting it negatively is easy enough: it means not lying, not hiding, and not dissimulating. (p. 112)
But Franz is enjoying the lies and hiding that comes along with his affair with Sabina. For Sabina, the idea of truth comes only when alone; the secrecy of the affair keeps it "truthful" because it is not given nor hidden from the public.
Franz, on the other hand, was certain that the division of life into private and public spheres is the source of all lies: a person is one thing in private and something quite different in public. For Franz, living in truth meant breaking down the barriers between the private and the public. (p. 112)
Despite this, Franz finds that he can no longer hide his love of Sabina, and tells his wife about his affair.
The Lost Children: A Charity Anthology