In this book, he shows that wealth, health and standard of living all tend to increase as population increases. The reason for this is very simple. Every time a child is born, the pessimists see an extra mouth to feed. The optimists see an extra brain, capable of solving problems. The world now has over six billion highly sophisticated, general purpose, self-programming computers in it, each of which is capable of solving problems, if it's allowed to.
People create resources.
They invent new products. They develop ways to use existing resources more efficiently, and they find ways to use resources that had previously been considered either useless or dangerous. The environment also benefits when people are allowed to become wealthy. People like clean air, clean water, and unspoiled landscapes, even if they have to pay to clean the environment up themselves. The worst ecological disasters have occurred in third-world countries and in totalitarian dictatorships.