"We have also a more sure word of prophecy." 2 Peter 1:19 The word for more sure here is bebaios, meaning firm, more sure, certain, binding, and steadfast.(1) Peter says prophecy is more sure, more certain, and more binding that eye witness testimony. Why?
One reason is that, disregarding the inerrancy of Scripture, typically eye witness testimony is considered a less than reliable body of evidence.
"Since the 1990s, when DNA testing was first introduced, Innocence Project researchers have reported that 73 percent of the 239 convictions overturned through DNA testing were based on eyewitness testimony. One third of these overturned cases rested on the testimony of two or more mistaken eyewitnesses."(2)
Eye witness accounts are highly susceptible to the passage of time, to preconceived beliefs, to the effects of fatigue, patience, and by "filling in the blanks" of information to speed processing/cataloging/retrieval of information. The term we used in practice was a form of generalization, however, currently University College London neuroscientist Karl Friston has expanded on the theory, calling it a process of "predictive coding."(3)
Our brains actively predict or guess what we are seeing when there is not enough information, or even when the image is first coming into our brains, predicting what is going to be visualized and as a result, the brain accepts that guess as an accurate representation - even if it is totally wrong.
Have you ever seen the video in which a man in an ape-suit runs across the court during a basketball game and people interviewed afterwards say they never saw the "ape," even though it ran right across their field of vision? Pretty funny. Here is a link to a site where you can watch an example of this phenomenon. http://www.livescience.com/6727-invisible-gorilla-test-shows-notice.html
A Stanford University paper lists the following about the accuracy of eye witness testimony:
"Accuracy of recollection decreases at a geometric rather than arithmetic rate (so passage of time has a highly distorting effect on recollection); accuracy of recollection is not highly correlated with the recollector's confidence; and memory is highly suggestible – people are easily ‘reminded’ of events that never happened, and having been ‘reminded’ may thereafter hold the false recollection as tenaciously as they would a true one." (4)
In addition, truthfulness, like beauty, is in the eye of the beholder. While in grad school, I performed an experiment to determine if good looks or race had any affect on one's perception of truthfulness. I showed a picture of an attractive, blond, white male, white female; black male; and black female; as well as unattractive versions of the same along with a vignette (short story) about the person. The participants were asked to decide if they thought the person's dialogue in the story was truthful or not.
In short, the experiment revealed that white, attractive males were perceived as the most truthful and honest, followed by attractive black males, then attractive white females, then attractive black females, and then all followed by unattractive subjects. Attractiveness counts, as does race and gender. Remember, this was all unconscious processing happening as far as I could observe. The results were consistent with similar studies done, so this was not relative only to my sample.
This is all interesting, but since Scripture is inerrant, I believe in this case, Peter had another reason for considering prophecy a more sure word: eye witness testimony requires at least two witnesses, and is even then questionable, but prophecy is objectively self-validating. It either comes to pass or it does not.
"When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that is the thing which the LORD hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him." [Deut.18:22 KJV]
"The prophet which prophesieth of peace, when the word of the prophet shall come to pass, then shall the prophet be known, that the Lord hath truly sent him." [Jer. 28:9 KJV]
You can't get better evidence than that.
1. Strong, James. The Strongest Strong's Exhaustive Concordance of the Bible. Grand Rapids: Zondervan, 2001
2. Arkowitz, Hal and Lilienfeld. Scott O. Scientific American, http://www.scientificamerican.com/article/do-the-eyes-have-it/. 2009
3. University of Glasgow. What our eyes can't see, the brain fills in. Phys.Org. http://phys.org/news/2011-04-eyes-brain.html. 2011
4. Engelhardt, Laura.The Problem with Eyewitness Testimony: a talk by Barbara Tversky, Professor of Psychology and George Fisher, Professor of Law. http://agora.stanford.edu/sjls/Issue%20One/fisher&tversky.htm. 2015 citing from Krist v. Eli Lilly and Co., 897 F.2d 293, 297 (7th Cir. 1990), (listing the findings of various psychological studies.)