If the stolen letter had been hidden anywhere within the limits of the Captain’s search, or in other words, had been hidden in a manner expected by the Captain, it would have certainly been found. The Captain, however, has been thoroughly mystified and his defeat lies in his assumption that the minister is a fool.
The Captain believes this because the minister has gained fame from his poetry.
The Captain said he thinks that all poets are fools, and thus he has fallen victim to his own prejudice.