 So, welcome to the session. In this session, we will try to see or try to review or try to understand the lecture given by Nobel Laureate Robert Solow on understanding the practical side of the sustainability. So, he has raised many dimension when it comes to the operationalizing sustainability or the practical side of it. Mostly we will take, we will discuss two questions. One, what obligation does one generation owe to others, whether it is ethical questions or whether this is a moral question. And second, to what degree are natural and other types of capital that is physical or human substitute, how long we can substitute or how much we can substitute. So, Professor Solow has given a very interesting way to understand the sustainability is sustainability is making sure that each successive generation is no worse up than the current one for the indefinite period of time. Now, first to understand whether this is ethical or moral obligation. So, the discussion goes like this, if it is moral obligation, so we should have a moral obligation to our future generation. But the fact is, it is unfavorable, it is undesirable. Why it is unfavorable? Unfavorable with the ground that we cannot, it is not that we should not be using mineral resources because we need to preserve this for the future generation. And undesirable that we will discuss in the next slide that why this can be undesirable. Obligation to conduct ourselves, so mostly rather than putting this in the ethical or the moral obligation, it is about the obligation to conduct ourselves so that we leave the future, the option or the capacity to as well as as we are. So, it is not about keeping something for the future generation, rather the way we behave, the obligation what we should have that, we should keep the option for the future generation or the capacity to the future generation to be as well up as we are. And why we should not preserve or keep anything for the future generation, because we do not have any idea about the taste, preference, what the future generation will be having. The typical example is that our past generation was not sure that what would be our and test preferences. So, similarly the logic applies to us also that we do not know about the taste preference of the future generation. And future much like ourselves attributing to them, inputting to them, possibly that is not the way what we should understand the need of the future generation. And he has given the idea or he has given the opinion that sustainability is not about preservation, it is important on the production side that what kind of input can be substituted for another when the particular inputs is not available. And ideally the policy decision should be like this that we are obligated to leave behind a generalized capacity, it is not resources, generalized capacity which does not mean only resources we should keep for the future rather we should keep the capacity to create well being not any particular thing or not any particular natural resources. Now, he has on that basis based on the capacity or based on the resources, natural resources or the ability, he has defined two types of sustainability. One is weak sustainability and second one is the strong sustainability. Now, what is weak sustainability? It holds that human made that is the constructed capital can effectively substitute for natural capital and the services provided by the ecological system. And it is developed from economic model of growth and technological change in the context of limited resources. So, according to weak sustainability, natural capital may be used up if we invest in the other form of capital that will support the well being. So, it is about if I am using the natural capital, if I am able to invest in view of that then I should be using natural capital. And according to weak sustainability, all the human made capital can substitute the natural capital. Strong sustainability says that certain ecosystem function cannot be replaced by human made capital and it must be maintained. So, it is not that all natural capital can be substituted by human made capital, certain thing cannot be replaced by human made capital, so few things we need to maintain. So, sustainable development is achievable as long as total natural capital that is K n plus the man made capital K m remain constant. Conventional economic view says that it is okay to reduce K n stock as far as they are being substituted by increase in K m stock. So, it is okay till the time we are reducing the stock of natural capital till the time we are able to substitute that in the form of the human made capital. And the rationale given over here is that increasing man made stock provide high income which leads to increase the level of the environmental protectionism. This is typically known as the substitutability paradigm. The criticism over here is that what about following substitution to maintain K m, K n remain constant. Can we invest in boat when we use more of fish? Can we use on pumps when you use more of aquifers? Can we use on, can we invest on sawmill when we use more of forest? Possibly no, this is what the criticism. You cannot substitute everything by investing in the human made capital. Then from there from the very weak sustainability because of this criticism we moved into weak that is the modified of solo sustainability. They say that S t is achievable by maintaining K n and K n constant only by preserving the non-substitutable proportion and component of K n stock. So, that part of K n which cannot be substituted can be preserved. And the rationale given over here is that upper limit on the non-substitutable proportion or the component of K n stocks are needed to preserve the biodiversity and ecosystem resilience to meet the human need. But the problem over this weak sustainability is that there is no scientific consensus over the set of physical indicator required to monitor and measure the biodiversity and ecosystem resilience. Typical example is that how much CO2 to be could be emitted. Now the strong sustainability says that sustainable development is achievable only when K n is constant. This is strong sustainability. This is based on the non-substitutable paradigm. The rationale given over here is that non-substitutable component of there would be always some component of K n. There is always uncertainty about the ecosystem functioning and their total service value. So, what is the total service value for the ecosystem is now we do not know whether the same value can be maintained over 20 years, 50 years or 100 years. There is some irreversibility of some environmental resource degradation and loss and scale of human impact relative to global carrying capacity. There is a scale effect works here. Typical example is greenhouse effect, ozone layer depletion and acid rain. So, we need to keep the K n constant then only we can achieve the strong sustainability. Let us have a pictorial explanation of this weak sustainability and strong sustainability. So, K t is our human made capital stock. A t is our natural capital stock. K t, A t the sum of capital is assets we have ignore all other form of capital. This is the total stock. Now to what extent we can substitute? So, we can do the investment from human made capital stock and we can do the consumption from the natural capital stock. So, K t plus 1 is equal to K t plus I t including the investment and this becomes production investment and then the production which is a function of K t and A t. And this natural capital stock is gets reduced when we use the consumption part of the natural capital stock, but also the stock renewal function gets added into the natural function because natural resources also get renewed over a period of time. The simple model of stock is that to what extent we can substitute. So, this depends on how much is the progress we are making or the typical PT over here which is exponentially increasing multiplier for the technological progress is it sufficient to overcome the depletion of the natural resources. So, the idea over here is that we can substitute certain portion of it, but to how much substitute and how much substitute which would be enough for the future generation that is not clear. So, Professor Solow, this is in the part of one of his lecture, he has tried to give the operationalization of the different sustainability definition and he has brought the concept of weak and strong sustainability, where weak sustainability says that the human made capital can replace whatever the consumption of the natural capital is being done, but strong sustainability says that it is not possible to invest in human made capital and get all the natural capital what is being consumed. Thank you.