In fact he was incurably insane and hallucinated more or less continuously, but...

In fact he was incurably insane and hallucinated more or less continuously, but by a remarkable stroke of lateral thinking his fellow wizards had reasoned that, in that case, the whole business could be sorted out if only they could find a formula that caused him to hallucinate that he was completely sane.*
*This is a very common hallucination, shared by most people.