Peter Sutton mentioned the following kind of case the other day, which I think is worthy of some consideration.
Zombie Brian is someone just like me who has no inner life. Some debate has ensued about whether Zombie Brian is a real possibility or not, much of that debate starting with the assumption that Zombie Brian can be clearly and distinctly conceived.
Pain Brian is someone just like me whose inner life consists of perpetual excruciating pain, on top of my normal feelings. If property dualism is correct, Pain Brian is a real, if immensely tragic, possibility.
But I don’t conceive of Pain Brian as easily as I conceive of Zombie Brian. I’m really not sure what it would be to be constantly in pain and acting just the way I act. I strongly suspect Pain Brian is a metaphysical impossibility.
Some I’m sure will have the intuition that Pain Brian is possible. Others will argue (not without reason) that even though Pain Brian is a real possibility, there are reasons to do with his ‘distance’ from real possibility that we could not conceive him. I just wanted to note that it is interesting that arguments from intuition like this don’t always point in the direction of dualism – sometimes they can point just as well towards physicalism.