The fundamental error of their matrimonial union; that of having based a permanent contract on a temporary feeling.

Hardy’s sentence distills a recurrent theme in his fiction: the mismatch between the legal/social permanence of marriage and the volatility of human desire. The “permanent contract” evokes marriage as an institution—binding, public, and difficult to undo—while “temporary feeling” suggests passion, infatuation, or even the shifting currents of affection that time and circumstance erode. The phrasing implies a cool, almost clinical diagnosis (“fundamental error”), aligning with Hardy’s often fatalistic view that individuals are trapped by conventions and by their own misread emotions. The quote can be read as both social critique and psychological observation: love may initiate commitment, but it is rarely stable enough to justify irrevocable vows without deeper compatibility and shared endurance.