 One of the aims of sustainability science is to guide social, economic and environmental policy. Now, that kind of actionable item comes with certain degrees of consequences. So, policy is something that needs to be measured. You need to be able to quantify and to characterize how far away you are from your ultimate goal. Quantifying and characterizing how far we are from attaining sustainability comes with many different challenges. One of the challenges is that because we need to be very pragmatic, we end up assigning values to different objects. So, whereas my comments a counter-intuitive thing to assign quantitative value to natural building, to assign a quantitative value to your experience in your neighborhood, you need to be able to measure those in order to determine whether or not you are approaching your goal, whether or not you are likely to succeed. So, you can see how it is very tempting to assign monetary value to different resources. And once you start assigning a monetary value to different resources, it is very easy to see how you can trade natural resources with man-made resources. So, this is the foundation of weak sustainability. And in weak sustainability, you are buying man-made capital with natural capital. Now, where that might be certainly easier to measure than something that is intangible, such as the natural beauty that you're observing in the background, it comes with a number of different proposals. So, when do you say that you can no longer buy man-made capital with natural capital? Where do you put the boundaries on how much you can use if you potentially could continue to produce man-made capital by putting ecosystems, by putting environmental resources under very extreme constraints? The other end member of weak sustainability is strong sustainability. And in strong sustainability, we don't start from the point of commoditizing individual environmental properties and individual environmental resources. We start by looking at the value of the environmental capital and how it is not easily or possibly replaced by man-made capital. So, a good place to start is talking a little bit about what it is that the environmental capital is. An environmental capital can be broadly divided into four different types of categories. The land, the atmosphere, the hydrosphere, and the biosphere. Clearly, these four categories are incredibly broad and include a wide variety of things. The atmosphere, for example, includes the climate. The hydrosphere includes a number of different things that have already been commoditized, such as hydropower, access to different reservoirs, groundwater, the quality of these waters, and the access to these waters. The land not only includes soils and their quality and fertility, but they also include mineral resources, geomorphic properties. Basically, are we dealing with flat land versus very hilly landscape, for example. And finally, the biosphere can be divided into an immense number of different things, from understanding the different links within an ecosystem to understanding the source of calories that they can provide and the genetic and pharmacological capital that they can provide. We are at Greenwood Furnace. That is a park that is in central Pennsylvania, and you can see several examples of natural capital being expressed. You have very large access to water, what you're not seeing right now. It's the reason why some of these structures were built, which was the extraction of iron ore supported by the presence of limestone. And finally, the access to a lot of biomass, basically trees to be able to make charcoal to run the furnace. And what you're also seeing is the fine interactions between the different types of capitals, and those kinds of interactions can include a variety of different things. And they include, of course, links within the ecosystem, but they also can include information, productions such as historical, research-wise, and educational information, et cetera, et cetera. So the fine links between the different types of economic capital make it even more difficult to try and substitute economic man-made capital with environmental capital.