Yup we are trying to weaken Lee's argument, which would in turn defend Kim's.
Lee is saying that Kim is probably wrong in her theory because people couldn't know their life expectancy was increasing while they were living. The implicit assumption he's making is that this kind of knowledge about one's increasing life span is highly unlikely.
But what if the effects of an increasing life span were manifested in other ways, so that the increase could be felt? Then Kim's conclusion that the optimism is from longer lives could be correct. This is what A provides us with. People could be aware that their life span is increasing if the effects of the increase are shown in better economic conditions, which in turn made people happy.
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Lee is saying that Kim is probably wrong in her theory because people couldn't know their life expectancy was increasing while they were living. The implicit assumption he's making is that this kind of knowledge about one's increasing life span is highly unlikely.
But what if the effects of an increasing life span were manifested in other ways, so that the increase could be felt? Then Kim's conclusion that the optimism is from longer lives could be correct. This is what A provides us with. People could be aware that their life span is increasing if the effects of the increase are shown in better economic conditions, which in turn made people happy.