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From the Middle Age through to the 18th century, the traditional division of the Eurasia into two continents followed Ptolemy with boundaries following along the Turkish Strait, the Black Sea, the Azov Sea, and the Don.
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However, a new convention emerged in the following centuries drawing the continental boundaries along the Tanais or the modern Don River. The convention was also followed by the Herodotus in the 5th BC. The boundary between Asia and Europe was placed along the Phasis River by Anaximandar. However, the Europe-Asia boundary remained unusual even among the Greek geographers and even Herodotus.
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The threefold system by Herodotus divided the old world into three: Europe, Africa, and Asia. However, the Greeks viewed the continent as physical entities with minimal cultural significance or political content. The two masses of land were separated by a complex waterway flowing from the Aegean Sea to the Sea of Azov. The original continental separation was invented by ancient Greek Mariners who named the continents as Europe and Asia. These transcontinental countries include Turkey, Russia, Kazakhstan, Georgia, and Azerbaijan. The modern definition of the Eurasian boundary places some countries into both continents while others entirely in one continent. The division of the two continents is purely cultural and tends to be biased towards the European side. As the ocean does not separate the two continents, they both exist on the same landmass. However, controversies exist over the exact boundary. The boundary between Asia and Europe is often defined to follow the Aegean Sea, the Caspian Sea, the Turkish Strait, the Black Sea, the Greater Caucasus, and the Ural River and Mountains. The boundary between the two continents is just merely a historical and cultural construct. However, there is no significant physical distinction between the two continents. The boundary between Europe and Asia exists on paper.