Tomas shrugged his shoulders and said, "Es muss sein. Es muss sein."

It was an allusion. The last movement of Beethoven's last quartet is based on the following motifs:

To make the meaning of the words absolutely clear, Beethoven introduced the movement with a phrase, "Der schwer gefasste Entschluss," which is commonly translated as "the difficult resolution."

This allusion to Beethoven was actually Tomas's first step back to Tereza, because she was the one who had induced him to buy records of the Beethoven quartets and sonatas.

The allusion was even more pertinent than he had thought because the Swiss doctor was a great music lover. Smiling serenely, he asked, in the melody of Beethoven's motif, "Muss es sein?"

"Ja, es muss sein!" Tomas said again.