Milena Popova, guest-blogging while Charles Stross is out experiencing Japan, has a long discussion up about public goods and why content (digitally speaking) is a classic example:
There’s a theory in economics about things called “public goods”. To understand the distinction between private goods, public goods and the couple of shades of grey in between, you first need to get your head around two concepts: rival and excludable.
Rival: (Wikipedia seems to call this “rivalrous”, but when I were a young economist lass we used to call it rival so I’ll stick with that.) A good is rival if my consumption of it diminishes the amount of the good that you can consume. Say we had 10 apples, and I ate one. There would now be 9 apples left which you could eat. If we had one apple and I ate all of it, tough luck, no apples for you. Knowing whether a good is rival or not tells you whether you want to use the market (if I were a good economist that would possibly be capital-M Market đ to allocate access to that good. If it’s rival, then the market is an efficient way of allocating the good; if it’s not, then you might want to think about other ways of getting your good to people. Remember that scary anti-piracy clip at the start of your DVDs which says “You wouldn’t steal a handbag”? Hold that thought for a minute.
Excludable: A good is excludable if you physically have a way of stopping people from consuming it. Back to the apples: if they’re in my fridge, inside my locked house and you don’t have a key, you can’t have my apples. (Yes, yes, you could break in. The law provides additional protection here, but ultimately there’s probably a better way for you to obtain an apple than breaking into my house, right?) Knowing whether a good is excludable tells you whether you can use the market to distribute the good. If your good is excludable, go ahead and sell it on the open market; if it’s not — you might struggle because you can’t stop people from just taking it for free.
So. Most of the goods you deal with in your day-to-day life are both rival and excludable. We call them pure private goods. But there’s a few things here and there that aren’t as clear-cut, and this is where it gets a little messy.
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