possible vs. impossible

[spoon]

Dear teachers,

Here is one more question! I hope it is the last one!

(a) It is possible to persuade him.
(b) It is impossible to persuade him.

(a') * He is possible to persuade.
(b') He is impossible to persuade.

I'd like to know why (a') is incorrect or ungrammatical.

[Mister Micawber]

.
I'm afraid that the only answer is that we use impossible as an adjectival attribute of people, but we do not use possible in that way:

My wife is impossible! This puzzle is impossible!
He is an impossible customer-- he is never satisfied.

[spoon]

Thanks anyway!