Jesus often justified His statements by appeals to authority. An appeal to authority can be a logical fallacy if the authority cited doesn't have credibility or if the authority is cited falsely. I once was talking to a woman who cited an historical fact which I knew to be false. When I told her that I understood the facts to be different, she told me, "Well, my son has a history degree and he told me that's the way it happened." Of course I did not argue further with this dear lady, but her argument was fallacious. First, having a history degree does not qualify one to know every fact of history, nor does my lack of a degree disqualify me from knowing this one. Second, I have no idea whether her son would represent his thoughts in this way or not.
However, appeals to authority need not be fallacious, and can provide valid evidence of varying degrees of certainty. To evaluate an appeal to authority we must evaluate, 1) the credibility of the authority, and 2) the accuracy of the citation. Many of Jesus' statements appeal to two authorities. One is the Scriptures of the Old Testament. The other is even more profound in its implications: He appeals to Himself as an authority.
Jesus often introduces His appeals to the authority of the Scriptures with the phrase, "It is written..." When He is tempted by Satan, Jesus uses the phrase to justify an ethical judgment. He quotes Deuteronomy 8:3, for example, when He says, "It is written, 'Man shall not live by bread alone, but by every word that proceeds from the mouth of God.'" (Matthew 4:4) This argument might be summarized this way:
All the Scriptures say is true.
The Scriptures say this.
Therefore this is true.
Jesus also cites the Scriptures when pointing out how His own life has fulfilled prophecies regarding the Messiah. What is interesting about these arguments is that He does not state the major premise nor the conclusion, but only the minor premise. These arguments can be analyzed in this way:
(Unstated assumption) The One who fulfills the Old Testament prophecies is Messiah.
I have fulfilled the Old Testament prophecies.
(Unstated conclusion) Therefore I am Messiah.
On the night of His betrayal, Jesus related the events to Psalm 41:9: He answered and said, "He who dipped his hand with Me in the dish will betray Me. The Son of Man indeed goes just as it is written of Him, but woe to that man by whom the Son of Man is betrayed! It would have been good for that man if he had not been born." (Matthew 26:23-24)
It is also important to consider Jesus' appeals to Himself as an authority. The Old Testament prophets, when they spoke words God had given them to say, would introduce such statements with the phrase, "Thus says the Lord." Jesus never used this phrase. However, He introduced many statements with a phrase variously translated as, "Verily, verily I say unto you," or "Assuredly I say to you." Jesus uses this phrase again and again to introduce prophecy, judgment, declarations of forgiveness, and doctrinal teaching. The implications of substituting "I say," for, "Thus says the Lord," is profound. Again, it is an argument with an assumed premise and an unstated conclusion.
(Unstated assumption) Only God can forgive sins. (Or judge, teach authoritatively, tell what the future will bring, give sight to the blind, etc.)
I forgive your sins.
(Unstated conclusion) Therefore I am God.
Consider this exchange from Mark 2:5-6: When Jesus saw their faith, He said to the paralytic, "Son, your sins are forgiven you." And some of the scribes were sitting there and reasoning in their hearts, "Why does this Man speak blasphemies like this? Who can forgive sins but God alone?" Of course, Jesus' statements would not be considered blasphemous unless it was assumed that Jesus' words and actions implied the previously stated argument.
These unspoken conclusions frustrated Jesus' enemies, who wanted desperately to have a clear-cut reason to stone Him. They knew the implications of Jesus' spoken words, and of His miracles, but wanted Him to say it outright. "Tell us plainly. Are you the Christ?" they demanded. He answered that He had told them, but they had not believed. (John 10:23-24)
Through Jesus' use of the Scriptures and of His own words as authorities, He sends us a clear message: The Scriptures are the true Words of God; they speak of Me; and I am of God.