In the proto-world, unoccupied land is generally regarded as held in the commons, free for use by hunters, fishers, herders, gatherers, migrations, ceremonies, scattering seeds of herbs and plants you like, and whatever else. There is basically no unclaimed land.

> Would you argue that you have no right to say "bugger off" to that person

You have no inherent right to do so, but it would absolutely be in your interest to do so. And indeed, while it took many thousands of years to settle the question, the settler-agricultural types did finally crush the more nomadic peoples in the last couple centuries and assert an absolute claim to formerly common property, so the question is moot: your property rights were established by thousands of years of violence and genocide by people tired of nomadic peoples asserting their traditional rights.

Fair enough, but how do you reconcile that with my point (3)? Police returning an item as in the given example means the police are thieves according to your definition.

If not, how do you propose mitigating this conflict?

In the same way as it is done today. Note that I'm not the one who is saying that land ownership is theft - you are, because you are trying so say "taking by (threat of) force is theft".

In reality, what is and isn't theft is defined by social convention. So land ownership, police returning items, and tax aren't theft, even though they all involve (the threat of) force. Taking an item from somebody else's home is theft, even when no force between humans is is involved at all.

I'm not saying that land ownership is theft. I don't advocate taking someone else's land by force, but I do claim that (at least in "proto-world") taking unclaimed land is acceptable exactly because it is unclaimed.

That whole discussion does lead to some other interesting points though... for example, if I abandon the home and garden I build on unclaimed land in proto-world, and move 8,000 miles away, does that land remain "mine" or does it return to its unclaimed state? It turns out that in the modern world, "we" decided that once you own a piece of land, it is yours forever, unless you sell it, trade it way, etc. I'm not actually convinced that is right. On this, I may be closer to the position of some left-anarchist types who hold that land ownership is temporary and only based in actual use/occupancy. I'm not entirely convinced one way or the other on this point though.

Inheritance is another interesting issue, although you can kinda sorta hand-wave around that by saying that "if you can transfer something to a new owner, then doing so a split second before you die is effectively the same thing as allowing inheritance".

In reality, what is and isn't theft is defined by social convention. So land ownership, police returning items, and tax aren't theft, even though they all involve (the threat of) force.

Sure, the social convention is what it is. What I'm arguing is that the social convention is wrong and needs to be updated. It's not like these things are chiseled into stone tablets and made immutable for all time.

I'll also point out that I'm far from alone on the "taxation is theft" thing. It may be a minority viewpoint, but it's hardly some fringe idea held by just two or three crackpots. There's at least five or six of us. :-)