Irreversibility 

            According to the second law of thermodynamics, the entropy (a measure of disorder or chaos) of an isolated system (one that does not exchange matter, energy, or information with its environment) will remain unchanged for reversible processes but will increase for irreversible processes. Thus irreversible systems, including all living systems, tend to become chaotic as their entropy increases. Nicolis and Prigogine (1989) speak of a “universal role of irreversibility in nature” and point out that irreversibility of complex systems is not the result of “the complexity of the collective behavior of intrinsically simply objects” but rather is “due to the very structure of the dynamical systems” (pp. 214-215). Irreversible processes that produce entropy are the creators of self-organization (Kondepudi & Prigogine, 1998).

            In chaos theory, systems changing over time are usually represented mathematically by evolution equations. These generally take the form of a series of differential equations that specify the rates of change of the system variables (Kellert, 1993). These systems undergo irreversible processes. 

            While the one-way aging of our body seems self-evident, psychology points out that our mind evolves and grows as well.  According to Jung (1978), our ego is “acquired” shortly after physical birth and develops over time much like the physical body. The ego “seems to rise in the first place from the collision between the somatic factor and the environment, and, once established as a subject, it goes on developing from further collisions with the outer world and the inner” (p. 5). In this sense, the ego is a dissipative self-generating or autopoietic system.

            Chaos theory views the future as a succession of probable events, and complex systems are attracted towards those possible states of maximum probability. Because complex systems, including living systems, as well as our Earth itself, undergo irreversible processes over time, the temporal direction from past to the future is called time’s arrow.  "The essence of time's arrow lies in the irreversibility of history" (Gould, 1987, p. 194).  The metaphor of time's cycles reflects nature's timeless laws rather than the contiguous moments of complex historical pathways. "Organisms follow time's arrow of contingent history; minerals, time's cycle of immanent geometrical logic." (Gould, 1987, p. 196).   According to Gould (1987), time's arrow and time's cycle join forces when we try to unravel nature's complexity.

            The idea of irreversibility should not be carried too far. While our physical body can sometimes be made to reverse its conditions and some of its characteristics, true reversibility, in the sense of actually going backward in time, is impossible. For example, If we are sick, we can recover our health and become well again; and, after a period of aging, we often can regain some of our youthful vigor through exercise and diet. But once we become an adult, we can never become a child again. 

            The march of history through time is associated with a dichotomy. At one end is time's arrow, an irreversible sequence of unrepeated events. At the other end is time's cycle, where apparent motions through time are parts of repeating cycles, and differences of the past will be realities of the future. Time's arrow is causal while time's cycle is acausal (Gould, 1987). According to Gould (1987), the concept of arrows and cycles of time lies deep in Western thinking; they may even be archetypal.

Psychologists often divide the human lifespan into stages. Erickson, Freud, and Piaget, for example, have all described various developmental stages that we go through in serial order. Jung (1989) describes three main stages or phases of life as: (1) the first years of life or presexual stage; (2) the second stage includes the years of childhood up to puberty; and (3) the third stage is the adult period from puberty on, and can be called the period of maturity. Harding (1973) also describes three stages: (1) the naive stage which addresses the urge to self-preservation; (2) the ego stage which addresses the urge to sexuality and parenthood; and (3) the stage of consciousness of the Self which addresses the will to power. All of these stages are irreversible, because the human body itself is irreversible as it grows and matures from youth to old age. Although the ego is irreversible, it appears to be possible to go backward in time. The human mind can pathologically temporarily revert to earlier stages in what is called regression. However, regression has more to do with the desire to focus consciousness and the sense of identity on a previously less stressful memory than an actual time reversal.