A Riddle about Doctrine

Here is a riddle.  I will give the answer in my next post.

One person teaches something to another.  I'll call them Jim and John.  Jim teaches John two things, call them doctrine A and doctrine B.  Both are true.  John respects and trusts Jim and wants to hold fast to both doctrines.  But it is impossible.  

There is no contradiction between doctrine A and doctrine B.  But no one can hold fast to both doctrines at the same time.  By "hold fast" I mean make and maintain a commitment to believe them.

John can hold fast to doctrine A but not doctrine B.  Or he can hold fast to doctrine B but not doctrine A.  Or neither. But he has to make a choice between them.  He cannot hold fast to both at the same time.

He has to choose between them.  Yet there is no contradiction between the two doctrines.  They are fully compatible with each other.  Both can be true at the same time.

No one can be committed to believe both doctrine A and doctrine B even though they are compatible with each other and there is no contradiction between them.  

Why?