If negotiations are to prove fruitful, there must not only be sincerity on each side, but there must also be . . . . . . . . in the sincerity of the other side.

The correct answer is: faith.

Faith is confidence or trust in something or someone. In the context of negotiations, faith in the sincerity of the other side is essential for a successful outcome. Without faith, it is difficult to build trust and cooperation, which are essential for reaching an agreement.

The other options are not as relevant to the context of negotiations. Certainty is the state of being sure about something. Substance is the quality of being substantial or important. Belief is an acceptance that something is true or real. While these qualities may be helpful in negotiations, they are not as essential as faith.

In conclusion, the correct answer is: faith.