A harbinger is a sign of things to come. Throughout history and literature, harbingers and omens figure prominently, and are responsible for major decisions which have altered the course of both.

If legend was fact, Christianity and Western civilisation could owe their existences to signs. The legend has it that Constantine the Great saw a vision in which either (depending on which version you're familiar with) a cross, a fish, an "XP" variant, or a slightly trodden wallaby was emblazoned on a soldier's shield. A loud, steady voice then allegedly told him, "By this sign conquer." Conquer he did, and shortly thereafter the Dark Ages overtook most of Europe and the Near East.

See also Inquisition

The actual mechanism of harbingers appears to be a quirk of human psychology. There are two general categories of symbolic prescience, both of which are subject to an individual's awareness of laws of Probability. Whether one is cognizant of the direction events in their life are headed, the subconscious keeps account of what is likely to happen in one's future. Depending on how aware a person is of these subtle instincts, their anticipation of imminent events will vary.

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 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.

In the Biblical sense, Prophecy, when describing the foretelling of an event, is to be subjected to scrutiny of the Scriptures, and may not be considered to be prophecy until the foretold events occur. Biblical prophecy is also distinct from harbingers, in the sense that the foretelling comes by the spoken word (often attributed directly to God), and not in the manifestation of signs.

Famous believers in harbingers: Abraham Lincoln, Shirley MacLaine, Hilary Swank; the Magi

"I am a great believer in luck, and I find the harder I work, the more I have of it." – Stephen Leacock.

"Harbinger" is also the title of the debut album by singer/songwriter Paula Cole.

"Harbinger" is also the name of a character in DC Comics' Crisis on Infinite Earths.

"Harbinger" is also the title of the episode 3.15 in Star Trek: Enterprise.