This is an absolute classic, and in my opinion it is up there with the the special theory of relativity and newtons theory of gravity. The reason is that feynman introduces one of those rare paradigm shifts that turns everything on its head: build from the bottom and up instead of the crudeness of building from the top down. The key insight being that atoms are like legos, and that it should be possible to make identical assemblers, based on the lego principle, that are able to replicate. Once this step is accomplished almost anything is possible.

Even now, almost 50 years later, the implications of what he started are only beginning to be felt - very few people even understand the implications of what he was proposing. He was so much ahead of his time it's unbelievable.