I believe with perfect faith that ... (Siddur).
There are Thirteen Principles of Faith whose absolute certainty we declare after the morning services. These are the only principles of absolute certainty. Everything else is subject to doubt. If there were anything else of absolute certainty, there would be fourteen principles.
Rabbi Issachar of Wolborz told his followers that the soul of a departed person had once come to him and stated that he was destitute and needed money for his daughter's wedding.
"But you are no longer alive," the Rabbi said to the soul, "and you have no need for money." The soul refused to believe him.
"How pathetic," the Rabbi said. "There are souls who are not privileged to enter Gehinnom (perdition) to undergo the cleansing process that will qualify them for Gan Eden (paradise). These lost souls may wander for years in a fantasy world, believing they are still alive."*
One follower asked, "How can we be sure that we are indeed living? Perhaps we too are in this fantasy world now, but are under the delusion that we are still alive."
The Rabbi answered, "People who consider it a possibility that they may be delusional are not delusional. Psychotics do not think for a single moment that they may be hallucinating."
The Thirteen Principles of Faith are axioms. With the exception of these, we should always be ready to examine our convictions, regardless of how strongly we may feel about them. It is when we are absolutely certain that we are right and have no doubt whatever about the validity of our opinions that we are most likely to be in error.
* In Kabbalah there is a concept of a "world of emptiness" where souls may dwell until they are cleansed.