The basic idea underlying the Principia Cybernetica Project is that evolution leads to the spontaneous emergence of systems of higher and higher complexity or "intelligence": from elementary particles, via atoms, molecules, living cells, multicellular organisms, plants, and animals to human beings, culture and society. This historical development can be understood with the help of the concepts: variety, constraint, variation, selection, fitness, system, and metasystem transition.
Complexity can be understood as a combination of variety and constraint. Variety is a measure for freedom or diversity, for the number of distinct possibilities or alternatives. Variety on its own leads to entropy, disorder and chaos. Constraint is what limits possibilities, by excluding certain alternatives. Constraint on its own engenders order, stability, rigidity. A system can be defined as a constraint on variety. If variety and/or constraint increase, the system's complexity increases.
Evolution can be understood as a combination of variation and selection, which are merely the dynamic, temporal versions of the static variety and constraint: variation produces variety, selection produces constraint. Evolution is governed by the principle that fit systems are selected, where fitness measures the likeliness that the system will maintain and reproduce. Unfit systems are eliminated, i.e. undergo variation transforming them into a different, possibly more fit system.
Fitness is relative: a system may be able to maintain in one environment, but not in another one. Thus a fit system can be said to "fit in" with its neigbouring system. The requirement of being fit is a constraint on the system: it excludes "unfit" behaviors. Thus evolution through natural selection tends to spontaneously create constraint or order.
In an indirect direct way it also tends to increase variety. Each newly evolved system defines a new environment or "niche" for other systems, to which they can fit, or not. Thus it opens new possibilities (variety) for being fit (constraint). The overall effect is an increase of complexity for the environment as a whole: more diverse systems are fitted together through more complicated linkages. This mechanism is exemplified by the increase of complexity in astrophysical evolution, chemical reactions, and ecosystems development.
Complexity also increases for individual systems: adaptation to a complex environment requires a large variety of possible actions, that compensate different perturbations (Ashby's Law of Requisite Variety). The more complex the environment, the more varied the perturbations, and the more varied the possibilities for counteraction need to be. The resulting increase of internal variety must be held in check by an increase of internal constraint, which selects the adequate counteractions. Otherwise, the system would need ever more time to select the correct counteraction. The only mechanism allowing unlimited increase of both variety and constraint is a metasystem transition (MST). An MST creates a metasystem: a constraint on a variety of object systems. Subsequent MST's lead to a hierarchy of control levels, where each higher level controls the constraints of the level below. The higher the number of metalevels, the more complex the system, and the more complex the environments to which it can adapt.
The MST concept makes it possible to reconstruct the sequence of evolutionary events from the beginnning of time to the present as a partially ordered series of metasystem transitions. These transitions can be roughly classified in four categories or "tracks":