Is there any knowledge on where the volitional ''おう'' Ending originated? As far as my interpretations go having seen a lot of sentences with them, Both the ''おう'' of the volitional and the ''おう'' of ’’こう、そう、どう'' seem to indicate some kind of..estimate? Something uncertain/imprecise. Instead of being 100% precise it's just a kind of general direction, a kind of estimate or expectation that is probably that way but not 100%.
それ is certain, it is that, そう is more vague, it is ''that way''. And it is used for ''seems like'' hearsay. 行く is just going. 行こう is more of an estimate, in that you'll ''probably'' go, or you say it when you say ''let's'' go, which isn't completely certain.
Another thing is how the other rows all start with the distance, and then some kind of thing that modifies it. like, その Is それ but possessive instead, so you use it in front of words.
Thing is, the こうrow has one that's ''ああ’’ which doesn't fall in line with this at all, but it may also be a sound change for all I know.
For all I know this could be a complete coincidence or my interpretation is completely false, soo, are they related in any way?