It is possible that declension suffixes once upon time were postpositions. At least I remember this was a theory behind Indo-European declension suffixes. If a

It is possible that declension suffixes once upon time were postpositions. At least I remember this was a theory behind Indo-European declension suffixes. If a postposition is used often enough after a noun, people quickly learn the pattern, and think of the pair as a single word. Although this must have happened before writing was invented, so probably people didn't think of what constitutes a word. Now every now and then it happens that a word dies in a language. A postposition could have died as a standalone word but was preserved as a declension suffix. And now we might only think of it as a suffix.