There are several arguments in metaphysics which are based on "conceivability":
- The ontological argument for God's existence.
- Hilary Putnam's Twin Earth argument for semantic externalism (the idea that there's more to the meaning of a term than the corresponding brain state of the person using the term).
- Saul Kripke's argument for dualism.
- Philosophical Zombies.
In each of these arguments, the starting point is the conceivability of a entity or property, and then a step is made from conceivability to possibility.
What I don't understand is the following: It is intuitive in several fields of inquiry that conceivability doesn't entail possibility in any way. So why is that step valid in the above examples? One could never build physical or metaphysical statements on the premise that faster than light speed is conceivable or that dragons exist.
My question: From a formal logic, what is exactly the difference between "XYZ water substitute is conceivable, therefore XYZ is possible" or "A difference between mental states and physical states is conceivable therefore Mental states are not the same as physical states" vs "Faster than light speed is conceivable therefore faster than light speed is possible" or "Fire regurgitating reptiles are conceivable therefore fire regurgitating reptiles are possible"? What is the logical principle behind the validity of the first two arguments but not the second two?