History [of ideas about free will]

Early in history it seems to have generally been assumed that everything about humans must ultimately be determined by unchangeable fate—which it was sometimes thought could be foretold by astrology or other forms of divination. Most Greek philosophers seem to have believed that their various mechanical or moral theories implied rigid determination of human actions. But especially with the advent of the Christian religion the notion that humans can at some level make free choices—particularly about whether to do good or not—emerged as a foundational idea. (The idea had also arisen in Persian and Hebrew religions and legal systems, and was supported by Roman lawyers such as Cicero.) How this could be consistent with God having infinite power was not clear, although around 420 AD Augustine suggested that while God might have infinite knowledge of the future we as humans could not—yielding what can be viewed as a very rough analog of my explanation for free will. In the 1500s some early Protestants made theological arguments against free will—and indeed issues of free will remain a feature of controversy between Christian denominations even today.

In the mid-1600s philosophers such as Thomas Hobbes asserted that minds operate according to definite mechanisms and therefore cannot exhibit free will. In the late 1700s philosophers such as Immanuel Kant—agreeing with earlier work by Gottfried Leibniz—claimed instead that at least some parts of our minds are free and not determined by definite laws. But soon thereafter scientists like Pierre-Simon Laplace began to argue for determinism throughout the universe based on mathematical laws. And with the increasing success of science in the 1800s it came to be widely believed that there must be definite laws for all human actions—providing a foundation for the development of psychology and the social sciences.

In the early 1900s historians and economists emphasized that there were at least not simple laws for various aspects of human behavior. But it was nevertheless typically assumed that methods based on physics would eventually yield deterministic laws for human behavior—and this was for example part of the inspiration for the behaviorist movement in psychology in the mid-1900s. The advent of quantum mechanics in the 1920s, however, showed that even physics might not be entirely deterministic—and by the 1940s the possibility that this might lead to human free will was being discussed by physicists, philosophers and historians. Around this time Karl Popper used both quantum mechanics and sensitive dependence on initial conditions (see also page 971) to argue for fundamental indeterminism. And also around this time Friedrich Hayek (following ideas of Ludwig Mises in the early 1900s) suggested—presumably influenced by work in mathematical logic—that human behavior might be fundamentally unpredictable because in effect brains can explain only systems simpler than themselves, and can thus never explain their own operation. But while this has some similarity to the ideas of computational irreducibility in this book it appears never to have been widely studied.

Questions of free will and responsibility have been widely discussed in criminal and other law since at least the 1800s (see note below). In the 1960s and 1970s ideas from popular psychology tended to diminish the importance of free will relative to physiology or environment and experiences. In the 1980s, however, free will was increasingly attributed to animals other than humans. Free will for computers and robots was discussed in the 1950s in science fiction and to some extent in the field of cybernetics. But following lack of success in artificial intelligence it has for the most part not been seriously studied. Sometimes it is claimed that Gödel's Theorem shows that humans cannot follow definite rules—but I argue on page 1158 that this is not correct.