If legend were fact, Christianity and Western civilization could owe their existences to signs. The legend has it that Constantine the Great saw a vision in which either (depending on which version we read) a cross, a fish or a Labarum monogram was emblazoned in the sky. A loud, steady voice then allegedly told him, In hoc signo vinces ("By this as your standard, you will conquer"). Conquer he did, bringing Christianity to the Empire.
An aware person will be able to explicitly imagine specific events, while someone who is less aware, or in denial, will not sense, or will ignore, the anticipatory signals their subconscious sends to prepare them in how they will respond to unfolding events.
Someone who is less than fully aware, but more than blindly ignorant, may encounter some sort of symbolic image which embodies the course of events their subconscious is trying to make them anticipate. Such an encounter may enhance the person's understanding, or in some other way make it easier for them to understand and accept the probability of unfolding events in their life. Subjectively, the person will have felt they had no previous intuition of events unfolding as the "sign" indicated. Should events then unfold accordingly, the person may be predisposed to believe in omens.
This first example relies on the harbinger encapsulating probable future events in the mind of someone who is not fully aware of what will probably happen. It does not necessarily resign the person to believing the future must happen accordingly, and important decisions are not based upon nascent foreknowledge. Also, it is likely that the person has experienced several unfulfilled harbingers, in which chance does not favour the occurrence of the "foretold" events. The mind may not record when signs fail to tell the future, leaving an artificial form of faith caused by selective memory of only those signs which could be seen to foretell events.
The second example of a harbinger occurs when an individual's faith in signs causes them to believe that a future event has been foretold. Here, the future is considered unchangeable, and the person makes important decisions based upon what they believe the harbinger has revealed it to be. In this case, the harbinger becomes a catalyst for the formation of a self-fulfilling prophecy. The laws of probability do not change in this instance, except that the individual alters the circumstances which make the occurrence of the foretold events more likely. Should the anticipated events occur, this is taken as proof of the validity of the sign.