Consulting the Oracle
A legendary case of advice that was not sufficiently specific occurred when the Croesus, King of Lydia, consulted the Oracle at Delphi. The Oracle, a priestess of Apollo, told him that a great nation would fall if he crossed the Persian border. Assuming that the great nation was Persia, he crossed and was defeated. It turned out the great nation that fell was his own. Of course, if he had won, the oracle could also have claimed to be correct. In reality, the Oracle told him nothing, but maintained her reputation for being able to see the future.
There wouldn't be a problem with the Oracle's prediction if it had been obvious to Croesus that her statement didn't tell him which side would win, so the vagueness itself was not the problem. Instead it was the vague statement together with the impression that the Oracle was telling him something of substance. The Oracle had no doubt earned her reputation for "accuracy" because her previous predictions had not turned out false. However, the previous prediction were most likely of the same type, such that any result could have been rationalized as being true.
It is important that we be able to recognize just how vague statements are, or else we too can be fooled into attributing great insight to sources that really are not telling us much. People who would like to appear wise are often very skillful at making vague statements that appear to have important content. Often this is done by people who genuinely believe in their own wisdom and have learned over a period of time how to phrase things so that they are successful in persuading others. Others people, like some phony fortune tellers, are fully aware that they are tricking people and make quite a bit of money doing it.
A theme commonly promoted by motivational writers and speakers is that you can achieve anything you want if you just try hard enough. Underlying this is an obvious truth that trying hard is quite helpful in achieving success. However inspirational leaders often go far beyond this by claiming that you can achieve almost anything. The vagueness comes in with the fact that how hard is "hard enough" is never clear. This statement can never fail. No matter how hard you tried, if you didn't achieve what you set out to do, you apparently didn't try "hard enough". Of course if you do succeed, you are likely to credit the motivator with giving wise advice.
A very similar situation might occur if a religious leader says that something you desire (like the recovery of a sick friend) will happen if your faith is strong enough. If it does not happen, that doesn't show that the leader's religious claims were wrong, it only shows that your faith must not have been strong enough. If you succeed, of course, you are likely to see the claim as being valid.
Politicians depend on vague statements to make themselves sound good while not providing anything specific enough for their opponents to refute. It is common to hear candidates claim they support the working man and woman, or that education will be a priority in their administration, or that they are strong believers in family values. These vague claims might give us the impression that this candidate cares about the same things we care about, but the reality is that anybody can make this kind of statement without any fear of being caught in a lie.
Appearance of supernatural abilities
Various psychics, fortune tellers, and mediums make a living by telling people things that seem like they could only have known by supernatural means. Sometimes this information is something already known to the customer which the psychic would not have any normal way of knowing. Other information is in the form of predictions or advice which seems to be based on knowledge of the future that the customer can evaluate after some time has gone by. Both types can be successful without involving actual supernatural phenomena when the claims are worded in a sufficiently vague way.
A case of predicting the future was discussed above where the Oracle of Delphi was assumed to have prophetic ability because her claims were phrased in such a way that they succeeded no matter what happened. Imagine the claim "a tall man with a beard will influence your life". How could this be proved false? Even if you never meet such a man, it is possible someone of that description helped pass a resolution in the city council that affects you, or that someone like that made a bad executive decision that caused a stock you own to lose money. Since predictions don't usually have a time limit, you could always assume this man hasn't influenced you yet but might sometime in the future. If you actually encounter such a man, you are likely to be impressed with the prediction, but when you don't, you are never in a position to recognize that the prediction was false.
Cold reading is the art of appearing to tell a person information about themselves without having any outside source of that information. The word "cold" is used to contrast it with "hot" reading where the psychic has secretly obtained some hard information about their customer. Cold reading is often employed by people who claim to be able to contact the dead, implying that the spirit of a dead person was their source of the knowledge. The reader will often use guesses that are made especially vague by prefixing an already vague claim with a phrase like "I sense something about..." or "I want to say..." so that the subject will be hard pressed to detect an actual mistake. Such a guess usually gets the subject to provide some confirmation and perhaps some additional information, which the reader will generally take credit for as if he or she knew it all along.
For example a reader might say "I see someone with a name beginning with the letter J". Of course there are many names beginning with J, the subject knows many people, and either the first or last name might begin with "J". Most likely, the subject will think of somebody with a "J" name and provide a little information the psychic can use.
Sometimes the reader might just ask a question, such as "Do you like pets?" If the person says yes, the reader says "Yes, I thought so," while if the subject says no, they reader can say "I didn't think so." Typically people will not remember the exact exchange, and if they like the idea that the reader has special powers, they will accept this as evidence of success. People who do this for a living are quite skilled at making vague and ambiguous suggestions that the subject is likely to accept as true, and at making maximal use of the feedback they get from the subjects. All but the most skeptical subjects can often be fooled this way.
Such psychics and mediums are not always frauds, although many are. People can pick up the habits of cold reading and learn to engage in exchanges that seem very successful while actually convincing themselves that that have a special gift. It is, of course, possible that some of them actually have psychic powers, but when what they can do is easily explained by cold reading with the occasional lucky guess, a supernatural explanation is not necessary.
Sometimes people can achieve successful cold reading type results using a system such as astrology or palm-reading. University of Oregon psychology professor Ray Hyman tried palm reading when he was young to make some extra money. The positive feedback he received from clients convinced him that the system must have some actual merit. That was until a friend convinced him to see what happened when he told each client the exact opposite of what the reading was supposed to be. He was amazed to find that the feedback from clients was as good or better than what he got when he gave the proper reading. Apparently the opposite of the vague claims could be fit to people's expectations just as well as the normal claims.
Vague claims can be interpreted so they fit almost any situation. If we mistakenly believe this fit is caused by the profound insight of the person making the claim, rather than the vague nature of the claim itself, we can adopt some seriously false beliefs. This could lead to spending a lot of money for advice from somebody who really has nothing useful to tell us, or taking bad advice, or even becoming involved in a some religious cult or some other dubious cause. Vague statements are a legitimate part of communication but it is important to recognize them for what they are.