Once accorded a high place in the world's mythologies, twins may be poised to reappear with new power in the images are archetypal patterns that shape the modern mind. On a concrete level, they are influencing how we define ourselves. As a result of intrauterine ultrasound studies, we are finding that more of us are twins than we ever imagined: a phenomenon dubbed "the twin without." We are also learning about within": our complementary "solar" and "lunar" traits of which the ancient Twin myths speak, whose integration is one of life's great tasks. Some of the most compelling messages of twins are appearing in an unexpected area: modern science. One of the great discoveries of 20th-century physics is the principle of complementarity: the idea that a subatomic particle can possess dual or twin aspects; that under certain experimental conditions it can manifest as a particle, and under others a wave. This is an expression of what could be called the "Twinship Principle" in modern science. The gene-environment relationship, which we have examined, is another. We might also add the predator-prey relationship in ecology; the alternation of day and night and the seasons; the sleep-wake cycle; perhaps even the one-and-zero relationship that is the basis of modern computer languages. In art, we find twinship in figure and ground, light and shade. Perhaps the most profound expressions of the Twinship Principle occur in psychology and spirituality: mind that is both local and nonlocal; the personal and the transpersonal; mortal and immortal; physical and immaterial; transcendent and immanent; sacred and profane. In all of these relationships, the entities illustrate the "coincidence of opposites." Like the poles of a magnet, they define each other and cannot exist without each other. Philosopher Alan Watts once said that the most profound metaphysical principle is that every inside has an outside, and every outside an inside. Is this the most important lesson of twins?