> You have to draw a line somewhere.

You don’t have to. We could just call them all islands. Greenland? Island. Eurasia? Island. Rhode Island? Not an island, strangely.

There are only between four and seven of these so called “continents” (we didn’t draw a line, we drew like five). They’re not popping up very quickly, either. I say we just forget the whole business came up.

And even if you just call them islands, you'll run into ambiguous territory somewhere. Is Manhattan really an island? If so, then is Harlem River really a river? (and there are also islands which become part of the mainland at low tide)

BTW, the state of Rhode Island was originally "the State of Rhode Island and Providence Plantations" which made sense but was unwieldy. Now the actual island of Rhode Island is called Aquidneck Island.

The trick to avoiding running into ambiguity is to notice the way that fish avoid running into drowning.

I think you are onto something.

Australia has 3x the land area as Greenland so the line is probably somewhere in between there.

True but it is all arbitrary. If there was an island twice as big as Greenland, what would it be called? And why is Europe considered a continent when it is more accurately just a peninsula off of Asia? And if we are talking seas, why is the Caribbean a sea when it is just a region of the Atlantic ocean? Even moreso for the Sargasso sea, which is a region right out in the middle of the Atlantic (it doesn't even have any land boundaries).

It's fuzzy rather than arbitrary, because the universe resists categorization. Australia is the dominant landmass on its tectonic plate, and it's relatively far away from other major landmasses. Greenland is a lesser landmass on the North American Plate, and it's much closer to the mainland.

Europe, Asia, and Africa are names people gave to the major regions surrounding the Mediterranean long ago. Later generations generalized the idea. First by using the same names for lands far behind the Europe, Asia, and Africa they were familiar with, and later by giving new names to separate landmasses. Whatever a continent is, Europe, Asia, and Africa are continents.

The main difference between a sea and a lake is that a sea is a part of the ocean. The Black Sea and the Baltic Sea are seas, while the Caspian Sea and the Great Lakes are lakes. The Caribbean Sea and the Sargasso Sea have names because people needed names for those regions. And because they consist of water connected to the ocean, calling them seas seemed appropriate.

Plate tectonics is a fairly recent 20th century theory; I believe Australia's classification as a continent predates it.

I think it might be considered a continent because people were looking for a continent. Since Roman times, there was reputed to be a continent at the South Pole called Terra Australis. Captain Cook was looking for Terra Australis, but found Australia instead. They new they weren't at the South Pole, but they decided to call the place Australia anyway (Antarctica would not be seen until several decades later.) I suspect the continent classification was cast as soon as they started calling the place Australia.

It can be fuzzy and arbitrary at the same time.

It's certainly arbitrary that the Gulf of Mexico isn't considered a sea. (or various other "parts of the ocean" referred to as "gulfs" or "bays")

"The main difference between a sea and a lake is that a sea is a part of the ocean."

Ok, so you have decided that the Caspian Sea isn't actually a sea? Well I guess all bets are off if you can do that.... :)

The Caspian Sea is usually considered to be the world's largest lake by surface area. It's also considered a sea in specific contexts, where the underlying motivation is clear. For example, a recent treaty defined it as a sea, dividing it into exclusive economic zones of the coastal states. As a lake, most of the offshore oil and gas reserves would have been shared equally between the coastal states.

Australia has its own tectonic plate. Greenland doesn't. I'm sure that helps.