The reason for which people are so reluctant to lump preposition and case ending together are numerous: first, the former come before and the latter after the form they govern; second, prepositions can be separated from their substantives by various additions such as an article and one or several adjectives, while case endings are permanently glued to the words they characterize; third, there is normally one preposition per phrase, irrespective of how many articles or adjective are added, whereas case-endings are likely to be found after every one of the additional elements; fourth, in the case of prepositions, the functional usually forms a clear-cut segment of the utterance, in contradiction to what we find, for instance in Latin case-endings, where the indication of case, i.e. function, is formally confused with that of a totally different type of moneme, namely number and where is not always clear what belongs to the substantive moneme, and what to the ending: is the nominative ending of puppis, ‘poop’, '-is' or just '–s' as in 'urbs?' […] But we should never allow them to blur the functional identity of prepositions and cases. All of them are finally reduced to the same phenomenon: monemes. Which for some reason or other, are frequently or constantly in contact and will tend to merge. This merging will be the more likely and the more intimate if the element whose function is indicated comes first, and the functional indicator last.
- Martinet (1962), a pag.45-46