There are things which must cause you to lose your reason or you have none to lose.

— Gotthold Ephraim Lessing, Emilia Galotti, 1772

Several translation variations from the original German can be found. Frankl doesn’t cite which translation he refers to (he may have been paraphrasing from memory) but I found another on English Stack Exchange.

The original in German,

Wahnwitzige? Das war es also, was er Ihnen von mir vertraute? —Nun, nun; es mag leicht keine von seinen gröbsten Lügen sein. —Ich fühle so was! —Und glauben Sie, glauben Sie mir: wer über gewisse Dinge den Verstand nicht verlieret, der hat keinen zu verlieren.

A madwoman? That is what he told you about me? —Well, well; maybe it’s not one of his grossest lies. —Something like it is what I feel! —And believe me, believe me: Whoever doesn’t lose his mind over certain things has no mind to lose.