If Descartes had accepted the coherence theory of truth, he would have had to have give up his quest for certain knowledge.

The coherence theory of truth states that a belief is true if it is consistent with other beliefs one holds. This means that truth is not absolute and certain, but rather dependent on the coherence of one's beliefs. If Descartes had accepted this theory, he would have had to give up his quest for certain knowledge as it would mean that knowledge is not absolute and certain, but rather relative and dependent on other beliefs.