[MUSIC] So, chapter five is all about evaluating and so evaluates sustainability in our systems and check our progress. There are properties to consider and methods to apply. We first talk about path dependence. And path dependence is when small initial differences in a system are a disproportionate cause of differences in outcomes or in other words, history shapes the present. So you talk about Lampard and me? And highways and railroads. >> Yeah. >> Can you explain those to me? >> Yeah. So, Frank Lampard and me, I parallel my life to an English professional soccer player who was born the same summer as me in 1978, and basically making the point that I became a professor, he became a professional football player like one that actually made enough money to be, to do that for his only job. But my dad was a professor, his dad was a professional football player. They grew up, he grew up in that environment, I grew up in this environment. So that's path dependence, the human level and then highways and railroads kind of this idea of path dependence with physical infrastructure, right? So once you've built this network of highways, you put into place, it's not going to be as feasible to use different forms of transportation, right? Or if you put in a really good network of railroads at the beginning, then it'll be harder to different forms of public transportations and railroads. So the basic idea is like, as we're evaluating our systems, which is what we want to do for sustainability and as we're creating new systems, thinking about like, okay, how long is this going to be in place for and what kind of decisions are we making for the long term future that are related to that. >> Okay. So gotta look at that complete path and how it's all dependent on the- >> Yeah, well I mean one thing is the decisions we're making today are based on decisions that have been made a long time ago, right, they're dependent on that path. But then also the decisions that we're making today are going to put in place another path going forward, right, so trying to be, pay as much attention to that as possible. >> Makes sense, the history shapes the present. >> Yes. >> [LAUGH]