As we know, most German vowels have a 'tense' (or long) pronunciation and a 'lax' (or short) pronunciation.
Most of the time, which pronunciation should be used can be determined by the context that that vowel appears in. Long vowels are doubled, followed by an H, or followed by a single consonant. Short vowels are followed by more than one consonant. E is pronounced short at the end of a word.
Of course, exceptions to this exist. In 'regnen' and 'Mund', the stem vowel is pronounced long even though the rules say it should be short.
But I was just thinking: are there any minimal pairs for the lax/tense distinction? The German wikipedia article mentions spuken vs spucken. But all the other minimal pairs it mentions are for consonants. There is of course the Mann vs man distinction, but I can't think of any others.
If there isn't minimal pairs for a vowel's lax/tense pronounciations, then why would they be considered separate phonemes? Without minimal pairs, they would be more akin to allophones, minus the fact that Germans are actually aware of the differences in pronunciation like they are for the different pronunciations of CH. German even has names for the two sounds of CH: the 'ich-laut' and the 'ach-laut' (Laut means 'sound').