Trust requires both freedom to betray and constraint making the relationship worthwhile.

Evidence

In conclusion, trusting a person means believing that when offered the chance, he or she is not likely to behave in a way that is damaging to us, and trust will typically be relevant when at least one party is free to disappoint the other, free enough to avoid a risky relationship, and constrained enough to consider that relationship an attractive option. — Diego Gambetta, Trust - Making and Breaking Cooperative…, 2729

Categorization

• DEFINE: Defines trust precisely as the conjunction of freedom-to-disappoint and constraint-toward-attractive-relationship — the exact operating definition read into Saramago.
• XREF: Directly grounds the reader’s thesis that the doctor’s wife, as sole sighted observer, supplies the constraint-and-risk structure under which trust becomes possible.