Suppose the placebo effects exists; if you believe you will get better, then you will indeed get recover.

B(h) -> h

Unfortunately, it only works if you believe you will get better - and it could be hard to see why it'd be rational to believe that in the first place. Fortunately, rationalists have a solution to this problem.

We're scientific sorts of people, so we believe in the placebo effect - that is:

B( B(h) -> h )

and we're also logical sort of people, so we believe lob's theorem

B( B(h) -> h ) -> B(h)

and hence we believe we'll be healed

B(h)

and hence, by the placebo effect,

h

And we're healed!