Chacun voit midi à sa porte is one of those expressions which, while being somewhat unfortunate, is nevertheless quite true. The literal translation is Everyone sees noon at his doorstep, which could translate as: Where you stand depends on where you sit, everybody has their own way of looking at things, or To each its own. Note that chacun (everyone) may be replaced by any noun or pronoun; il(s), elle(s), tu, vous, X, Y, Z etc..
At a basic level, the expression is used to state that we are all different, that what might hold true for one person might not hold true for another. Most often, however, there is the added connotation that we all are occupied, first and foremost, with our own personal interests, and see our subjective opinions as objective truths - which admittedly may get in the way of achieving a common goal. Now, while we may all agree that there is a fair amount of wisdom in the above allegation, we may also wonder: what has noon to do with it?
Centuries ago, before wristwatches and smartphones, the Egyptians and the Greeks had generalized sundials to tell the time, in particular that of prayer. In order to have precise sundials, one needed specialized gnomonic, trigonometric and cosmographic knowledge, and it took years of training to become a skilled and respected dial maker.
Throughout the Middle Ages, sun dials were installed above house doors in all large European cities. When you wanted to know the time, and on condition that it was at least a little sunny (a small, unavoidable constraint,) all you had to do is stick your head outside your front door and look at the dial. If it was noon, you saw noon at your door.
As the use of sundials spread to the countryside, however, the art of dial making started to loose in precision and craftsmanship, and the telling of time became more and more relative to the particular sundial installed above one's door. And that's when problems started: when it might be noon at your door, it might be not quite noon, or just past noon a couple of houses down the block. In other words, what you saw was not quite the same as what your neighbor saw. And it all started because of a few incompetent dial makers....
En dépit de leur volonté de collaboration, chaque état continue de voir midi à sa porte.
Despite wanting to work together, every state continues to act according to its self-interests.
Nous ne pouvons pas tous penser pareil. C'est naturel: chacun voit midi à sa porte.
We can't all think in the same way. Naturally, we all have different experiences and perspectives.