The old Prussian border with Poland came to an end in 1918/23, nearly a century ago, with the loss of the Provinces of West Prussia, Posen and eastern Upper Silesia. The border once again shifted westwards with the loss of East Prussia, eastern Pomerania, Mark-Brandenburg, Lower Silesia and western Upper Silesia. If you look at a modern railway network map of Poland today you can still discern the old imperial boundary between Prussia and Russian Poland because of the extensive railway construction on the Prussian side and sparse Russian construction in the east:

Such physical 'borders' are easily explained. But what is more mystifying is when human behaviour demarcates along an ancient border when the people living on both sides of the border are all the same now (Poles). The map below shows recent election results in the Republic of Poland with a very clear separation between the Free Market Party on the Prussian side of Poland and the Populist Parties on the Poland Proper side. This difference cannot be explained on the basis of two different national temprements - Polish and German - because the Germans are no longer on the western side of the line. If it were a matter of Russian influence in the east and German influence in the west, then one would expect the line to roughly bisect the country and not follow the curvature of the old border.

Doubtless interesting physical and social explanations could be given but in the end we are left guessing. Could this be some psychic or spiritual dividing line that somehow demarcates an unresolved injustice? It is interesting that the Populist (Pan-Polish) area is to the east in 'Poland Proper" (the former Russian and Austrian colonies) whereas the Free Market side is in the west. Who knows. I just don't think that this is a coincidence.