Paul Davies's book The Goldilocks Enigma (2006) reviews the current state of the fine tuning debate in detail, and concludes by enumerating the following responses to that debate:
1. The absurd universe
Our universe just happens to be the way it is.
2. The unique universe
There is a deep underlying unity in physics which necessitates the universe being the way it is. Some Theory of Everything will explain why the various features of the Universe must have exactly the values that we see.
3. The multiverse
Multiple Universes exist, having all possible combinations of characteristics, and we inevitably find ourselves within a Universe that allows us to exist.
4. Creationism
A creator designed the Universe with the purpose of supporting complexity and the emergence of Intelligence.
5. The life principle
There is an underlying principle that constrains the universe to evolve towards life and mind.
6. The self-explaining universe
A closed explanatory or causal loop: "perhaps only universes with a capacity for consciousness can exist." This is Wheeler's Participatory Anthropic Principle (PAP).
7. The fake universe
We live inside a virtual reality simulation.
