 Welcome back. We will continue our discussion of history. The question is how did the laws of thermodynamics originate? Actually how do the laws of physics originate? Thermodynamics is a part of physics. So let us look at the history of the laws of physics. See curious humans always studied nature and that is why parts of science were earlier known as natural science and natural philosophy. It was essentially a study of nature. And our understanding of study of nature led to some generalizations using inductive law. And these generalizations are today known as laws of physics. Of course, we are looking at physics. Other branches of science will have their own laws but the general scheme is essentially this. Even in physics if you look at for example, if you look at mechanical aspects, movement of bodies, acceleration, deceleration, gravitation, you finally end up with Newton's laws of mechanics. Whereas if you look at thermal aspects, you end up with laws of thermodynamics. But we know that nature is complex. And hence even if we look at just the energy and thermal aspects, the way we look at the nature, the behavior of nature, different people will generalize it in different ways. And over the years various people looked at it and generalized it in different ways. And hence the laws of thermodynamics also have the status that there are different ways of stating them. In fact, even different order in which they are explained and studied. These are known as different formulations of the laws of thermodynamics. For example, Joule proposed the first law in one way, Gibbs proposed the first law in another way. So, you can say that there is a Julian formulation of thermodynamics. There is a Gibbsian formulation of thermodynamics. When it comes to second law, we have a Kelvin's formulation of thermodynamics. So, we have Joule's formulation, we have Gibbs formulation, then we have Kelvin's formulation. Each one has its advantages and disadvantages. The modern formulations belong to a few people. A very significant name is Constantine Karatheode, a French-German scientist-mathematician. His formulation of thermodynamics based on mathematics and geometry is what we are going to use for the first law of thermodynamics. He has also a very nice formulation of the second law of thermodynamics. Unfortunately, that is too complex in mathematics for us engineers to understand. So, when it comes to the second law of thermodynamics, we will be using the Kelvin's formulation for the second law. Apart from that, there is a formulation by a mathematician Joule's which is an excellent formulation in the discrete domain. So, today we can say that the two major formulations of thermodynamics are those by Karatheode and Joule's. Thank you.