Extraordinary claims require
extraordinary evidence.

      The phrase "Extraordinary claims require extraordinary evidence" was popularized by Carl Sagan (1934 - 1996), a well-known astronomer and author who hosted a TV series called "Cosmos," published hundreds of scientific articles, and was professor of astronomy at Cornell University in New York.  The statement is self explanatory; if someone makes an extraordinary claim, there better be extraordinary evidence to back it up.  If, for example, someone made the claim that an alien race has made contact with earth, we would need sufficient evidence to verify the claim, such as an alien space craft, or an actual alien.  The extraordinary claim would need extraordinary evidence.
     At the heart of "extraordinary claims require extraordinary evidence" is a healthy and normal skepticism.  There are far too many charlatans and con-men in the world who make extraordinary claims without evidence to back them up.  Unfortunately, too many people lack the necessary skepticism and critical thinking skills to help them avoid being duped by con artists and wild theories.  Personally, except for a few qualifications, I agree with the sentiment of the statement "extraordinary claims require extraordinary evidence."  Those qualifications follow.

Presuppositions

     Requiring extraordinary evidence for extraordinary claims sounds good on the surface.  But, it is subjective.  The fact is that a person's presuppositions strongly affect how and to what degree the statement is applied.  In Jesus' resurrection, for example, Christians presuppose that God exists and that He could easily have raised Jesus from the dead.  The evidence of fulfilled prophecy, eyewitness records, and changed lives of the disciples is enough to convince many people who believe in God that Jesus rose from the dead.  This is a logical conclusion based on the presupposition and the evidence.
     Atheists, on the other hand, would negate the resurrection by default since their presupposition that there is no God¹ would require that God involvment cannot occur.   Therefore, for an atheist the extraordinary evidence would have to be "exceptionally" extraordinary in order to overcome his atheistic presuppositional base.   In other words, evidence would need to be presented that was rock solid and irrefutable. 
     This is why the skeptic must require "extraordinary evidence."  It enables him to retain his presupposition should the extraordinary level of the evidence not be met.  Therefore, requiring extraordinary evidence effectively stacks the deck against the claim. 

What would qualify as extraordinary evidence?

      When debating skeptics, I often ask them to tell me what would qualify as extraordinary evidence for the resurrection of Jesus.  Generally, nothing sensible is offered.  Normal evidence would be written accounts.  Extraordinary evidence would be a film, but we know that this extraordinary evidence is not reasonable.  Therefore, can the requirement that extraordinary claims (Christ's resurrection) require extraordinary evidence apply to Jesus' resurrection?  It would seem not.  Since Jesus' resurrection is alleged to be a historical event, then it seems logical that normal historical evidence and normal historical examination of that evidence would be all that we could do.  The resurrection is supposed to be an event of history and since it claims historical validity, then typical criteria for examining historical claims should be applied.

What criteria do they use to determine what is extraordinary evidence?

     The reality is that there is no precise scientific method for determining the validity of historic events.  There is a degree of subjectivity involved.  Different people will claim different requirements for validating ancient phenomena based upon their presuppositions and the type of evidence involved.  Also, since ancient events dealing with human history and claims cannot be observed or repeated, we must look at the evidence differently.  This makes the application of "extraordinary claims require extraordinary evidence" somewhat subjective and invalid for determining ancient phenomena.

Is the criteria for extraordinary evidence reasonable?

     The skeptic often requires "proof" that God exists, or "absolute proof" that Jesus rose from the dead.  I have heard many atheists, for example, say that the only proof they would accept of Jesus' resurrection would be if it could be tested using the scientific method.  Of course, we know that is an impossibility since the scientific method means observation, experimentation, and repetition and we can't apply that to an event that occurred 2000 years ago.  Atheists know this and that is why they require it.  When the Christian fails to produce a scientific method or scientific evidence, the atheist feels safe in his position.
     However, the requirement for absolute proof ignores the fact that there is a category of "sufficient evidence."  In logic, there is deduction and induction.  Deduction is drawing a conclusion based on facts.  It is reasoning from the general to the specific.  Induction is process of drawing general principles from specific facts.  It is from the specific to the general.  Often times, we use deductive and inductive reasoning to arrive at conclusions about events in history.  In so doing, there is no requirement of "extraordinary evidence."  The evidence is simply examined contextually; that is, it is examined according to the genre in which it fits.  This is what I mean...
     We do not apply the logical requirement of establishing a fact that is thus established through experimentation and repetition to the subject of Napoleon's existence.  The genre, history, does not fit that methodology.  Yet, the skeptic will sometimes require that experimentation and repetition be applied to Jesus' resurrection, thereby, misapplying evidential and logical analysis.
     Furthermore, we cannot ascertain all things with absolute certainty.  We cannot, for example, prove that Alexander the Great (356-323 B.C.) ever lived by observing him.  But, we have ancient writings from eyewitnesses concerning his existence.  Skeptics readily believe in Alexander the Great without involving the scientific method and without requiring "extraordinary evidence."
    However, a skeptic might say that Alexander the Great never claimed to have risen from the dead and that normal evidence would be sufficient to determine his existence with a reasonableness of probability.  But, Alexander the Great, according to history, performed an extraordinary feat.  By the age of 33 he had conquered the known world.  That is indeed an extraordinary event in history.  So, I ask, "Where is the extraordinary evidence to back that extraordinary claim up?"  Has any skeptic in Christ's resurrection, equally applied the principle of "extraordinary claims require extraordinary evidence," to Alexander the Great's conquest of the known world?  If not, then this brings us full circle to the issue concerning presuppositions.  With an atheist, for example, the presupposition that God does not exist means that the extraordinary claim of Christ's resurrection requires extraordinary evidence but Alexander the Great's world conquest does not, yet both are extraordinary claims of history.

Conclusion

     If it is true about Alexander the Great, no big deal.  It won't have any effect on anyone and it won't change anything in anyone's life outside of just having the information that he conquered the known world by age 33.  But, if it is true about Jesus, then that is completely different.  Jesus claimed to be divine and He had a message for people about heaven and hell and that salvation is only through Him.  Such a claim requires extraordinary evidence, such as a resurrection from the dead.  This would have a profound effect on people and it can make them uncomfortable.  Therefore, people will not want it to be true and will desperately try to hold onto their presuppositions; hence, the claim that extraordinary claims require extraordinary evidence.
     Nevertheless, when defending the Bible and dealing with the claim that "extraordinary claims require extraordinary evidence," address the following issues.

1. Will their presuppositions allow unbiased examination of the evidence?

2. What would qualify as extraordinary evidence?

3. What criteria is used to determine what is extraordinary evidence?

4. Are criteria for extraordinary evidence reasonable?

    Hopefully, a healthy dialogue can be had by both parties.