Thomas Aquinas

“ Persons are said to be unlawful subjects for marriage through being contrary to the law whereby marriage is established. Now marriage as fulfilling an office of nature is established by the natural law; as a sacrament, by the Divine law; as fulfilling an office of society, by the civil law. Consequently a person may be rendered an unlawful subject of marriage by any of the aforesaid laws. Nor does the comparison with the other sacraments hold, for they are sacraments only. ”

“ Further, no impediment can remove from a thing that which is part of its definition. Now indissolubility is part of the definition of marriage. Therefore there cannot be any impediments which annul a marriage already contracted. ”

“ But the causes which of their own nature corrupt a certain good are directed to that effect, and determinate, even as are the causes which produce that good; for the causes by which a thing is destroyed and those by which it is made are either contrary to one another, or the same but taken in a contrary way. ”

“ On the other hand, those impediments which regard the essentials of marriage make a marriage invalid, wherefore they are said not only to hinder the contracting of marriage, but to dissolve it if contracted ”

“ The conditions of particular persons taken individually are infinite in number, but taken in general, they may be reduced to a certain number; as instanced in medicine and all operative arts, which consider the conditions of particular persons in whom acts are. ”

“ Here, however, he is treating of the impediments as arising from the contracting parties, and these may be differentiated as follows. A person may be hindered from contracting marriage either simply, or with some particular person. If simply, so that he be unable to contract marriage with any woman, this can only be because he is hindered from performing the marriage act. ”

“ Wherefore, in order to afford an opportunity of proficiency towards a greater good, more impediments are assigned to matrimony than to the other sacraments.
Reply to Objection 2: The more perfect things can be hindered in more ways, in so far as more conditions are required for them. And if an imperfect thing requires more conditions, there will be more impediments to it ”