**Probability**

Probability is the measure of the likelihood that an event will occur. Probability is quantified as a number between 0 and 1 (where 0 indicates impossibility and 1 indicates certainty). The higher the probability of an event, the more certain that the event will occur. A simple example is the tossing of a fair (unbiased) coin. Since the coin is unbiased, the two outcomes (“head” and “tail”) are both equally probable; the probability of “head” equals the probability of “tail”. Since no other outcomes are possible, the probability is 1/2 (or 50%), of either “head” or “tail”. In other words, the probability of “head” is 1 out of 2 outcomes and the probability of “tail” is also 1 out of 2 outcomes, expressed as 0.5 when converted to decimal, with the above-mentioned quantification system. This type of probability is also called a priori probability.

**Probability theory**

Probability theory is the branch of mathematics concerned with probability, the analysis of random phenomena. The central objects of probability theory are random variables, stochastic processes, and events: mathematical abstractions of non-deterministic events or measured quantities that may either be single occurrences or evolve over time in an apparently random fashion.

It is not possible to predict precisely results of random events. However, if a sequence of individual events, such as coin flipping or the roll of dice, is influenced by other factors, such as friction, it will exhibit certain patterns, which can be studied and predicted. Two representative mathematical results describing such patterns are the law of large numbers and the central limit theorem.

As a mathematical foundation for statistics, probability theory is essential to many human activities that involve quantitative analysis of large sets of data. Methods of probability theory also apply to descriptions of complex systems given only partial knowledge of their state, as in statistical mechanics.

**Probability theory and Quantum mechanics**

Probability theory is required to describe quantum phenomena. A revolutionary discovery of early 20th century physics was the random character of all physical processes that occur at sub-atomic scales and are governed by the laws of quantum mechanics. The objective wave function evolves deterministically but, according to the Copenhagen interpretation, it deals with *probabilities of observing*, the outcome being explained by a *wave function collapse* when an *observation* is made.

**Etymology of the word probability**

The word probability derives from the Latin probabilitas, which can also mean “*probity*“, *a measure of the authority of a witness* in a legal case in Europe, and often correlated with the witness’s nobility.

From the point of view of idealism this makes really sense, because a observer has indeed a certain authority over the observed system, where the probability is changed by the act of observing.

**See also:**

– Quantum Zeno Effect and the Burning Bush