 So, the next thing we would like to look at in terms of resources, because it's important for our survival as a species and other species on this planet, is a renewable versus non-renewable resource. Now, there are some things that are overtly renewable. For example, we can rely, as far as we know, on the sun supplying us, radiating us, supplying us with energy for at least five billion more years. None of us are going to be around when, finally, the sun reaches the end of its life. And stars, as you probably know, reach the end of their lives at different times depending on their size and mass and the rate of burning their fuel. But we can rely on the sun being around as a potential source of light and also, of course, energy. Light we can convert various ways to a form of energy through our high-tech capabilities. So the sun is a potentially renewable resource. But they're also, and of course it's become very important to us, non-renewable resources. Are there things that we use faster than they can be replaced? And I'd like to talk about that for a while. Let's look at the non-renewable resources, which of course figure largely in our minds. There are ways in the past that we have depleted a stock or a material or an animal faster than it can be replenished naturally. That would include things like bison on the Great Plains in North America. For at least 10,000 years, the indigenous people in North America lived in some kind of equilibrium with the bison on the plains. They killed them. They ate them. They used their skins for warmth for their accommodation. They lived side by side with the bison without apparently destroying the viability of the herds which were roaming the Great Plains. When people came along and started shooting them, it was remarkable how quickly that stock of bison was depleted. Now, a lot of that shooting was done for no great reason. It wasn't done to supply food. It wasn't done for any great profound need. But certainly, it was dramatic in that the bison population fell dramatically. There are examples like that. Fundamentally, that's the real issue here. So let's just talk about that. So we need the stock replenishment rate and depletion. If all of these are in balance, we've got something of a renewable resource. As our resource, the stock is, for example, the number of bison around. If they naturally replenish at the rate that they achieve without any interference from humans, that doesn't mean there's no interference from other sources like disease or predators that they would encounter before Homo sapiens arrived on the Great Plains. As long as the replenishment is sufficient and overwhelms the depletion or is in some kind of equilibrium, the stock remains at some kind of equilibrium level. When you look at this, in fact, we're right into what is known as predator-prey theory, and that's a long-running, intellectually fascinating line of investigation in the natural world and other environments as well. So predator-prey theory has a strong mathematical basis now. The equations are known in terms of these kind of parameters that are written here.