Popper wrote extensively on the problem of determinism and free will, researched many earlier thinkers on the subject, and formulated his own "evolutionary" model of free will. In his Arthur Holly Compton lecture, Of Clouds and Clocks, delivered at Washington University in St. Louis in April 1965, he noted that earlier thinkers had seen the only alternative to determinism as chance. This is the classic argument against free will, that it comes to a stark choice between determinism or indeterminism. Hume found nothing between chance and necessity, and Eddington had said, there is "no halfway house" between randomness and determinism. But note that Popper may have been inspired by Arthur Holly Compton himself, who said an "act of choice [adds] a factor not supplied by the physical conditions...determining what will occur." See Compton's Atlantic Monthly article. In his dialogues with John Eccles, (The Self and Its Brain, 1977), at first Popper dismissed quantum mechanics as being no help with free will, but later describes a two-stage model that parallels Darwinian evolution, with genetic mutations being probabilistic and involving quantum uncertainty.
Popper replying to John Eccles:
"First of all, I do of course agree that quantum theoretical indeterminacy in a sense cannot help, because this leads merely to probabilistic laws, and we do not wish to say that such things as free decisions are just probabilistic affairs. "The trouble with quantum mechanical indeterminacy is twofold. First, it is probabilistic, and this doesn't help us much with the free-will problem, which is not just a chance affair. Second, it only gives us indeterminism, not openness to World 2 [Popper's Mind World]. However, in a roundabout way I do think that one may make use of quantum theoretical indeterminacy without committing oneself to the thesis that free-will decisions are probabilistic affairs.Here Popper compares new ideas to variation in the gene pool due to random mutations followed by natural selection, as William James had done in 1880.
"New ideas have a striking similarity to genetic mutations. Now, let us look for a moment at genetic mutations. Mutations are, it seems, brought about by quantum theoretical indeterminacy (including radiation effects). Accordingly, they are also probabilistic and not in themselves originally selected or adequate, but on them there subsequently operates natural selection which eliminates inappropriate mutations. Now we could conceive of a similar process with respect to new ideas and to free-will decisions, and similar things.
A few years earlier, Popper had called for a combination of randomness and control to explain freedom, though not yet explicitly, as is needed, in two stages with random chance before a controlled decision.
"freedom is not just chance but, rather, the result of a subtle interplay between something almost random or haphazard, and something like a restrictive or selective control" (Objective Knowledge, Of Clouds and Clocks, 1972, p. 232)
For those extreme libertarians who prefer a traditional definition of freedom as pure chance (liberum arbitrium indifferentiae), we can offer free will as Popper's interplay between chance and control (based on Compton's simple view of "chance, then choice"). In 1977 Popper gave the first Darwin Lecture, at Darwin College, Cambridge. He called it Natural Selection and the Emergence of Mind. In it he said he had changed his mind (a rare admission by a philosopher) about two things. First he now thought that natural selection was not a "tautology" that made it an unfalsifiable theory. Second, he had come to accept the random variation and selection of ideas as a model of free will.
The selection of a kind of behavior out of a randomly offered repertoire may be an act of choice, even an act of free will. I am an indeterminist; and in discussing indeterminism I have often regretfully pointed out that quantum indeterminacy does not seem to help us;1 for the amplification of something like, say, radioactive disintegration processes would not lead to human action or even animal action, but only to random movements. I have changed my mind on this issue.2 A choice process may be a selection process, and the selection may be from some repertoire of random events, without being random in its turn. This seems to me to offer a promising solution to one of our most vexing problems, and one by downward causation.Six years later, Popper wrote some very insightful and critical remarks on knowledge in lower and higher organisms. As we argue in information philosophy, biological information processing systems use their knowledge to achieve their purposes, primarily advancing themselves and their species in the struggle for survival.
The aggregation of human knowledge embodied externally - "exosomatically" as Popper puts it - is the core idea of the All organisms are professional problem solvers: before life, problems did not exist. Problems and life entered the world together, and with them problem solving. At first, the problems were sheer survival problems: an organism was, at first, just a professional survivor. In time this changed, and a problem today may be for some people a vastly different thing: it may be "How did life originate?" or perhaps "In which of his dialogues, if in any, does Plato attempt to portray Socrates?" - this is obviously not a survival problem. The problems may be created by the clash of the organism with its environment. But the tentative solutions, the trial and the errors all come from, the organism. They are all a priori, even though the environment constantly plays its part, of course. The view which I am trying to make attractive to you is, I am afraid, utterly different from those views which are at present held by almost everybody and which I have described, for the past 60 years, as the "bucket theory of the mind" (die Kübeltheorie des Geistes). But today an inventor theory of the mind and a discovery theory of the mind and a searchlight theory of the mind ought to be a little more acceptable than they were 60 years ago... I find it of the greatest importance to get over the myth (for it seems to me a myth) that anything starts with a stimulus and the ability to respond to it. Everything, I suggest, starts from the organism and its eagerness, its need, which seeks for anything it can use as a message, as information or as a so-called stimulus: I am not quarrelling about words, I am insisting upon the obvious fact that it is not its capability of being stimulated which makes the organism, but the exploratory apparatus, the keenness to respond, which makes the stimulus. It is the organism which, through many trials over millions of generations, learns to respond to this kind or that kind of stimulus: it turns something into a stimulus; it invents the ability to "see" this or that, but it may do all this a priori, by a mutation which, in its turn, may be due to its mutationally acquired mutability. I am a Darwinist and I am not a vitalist or a Lamarckist. I agree that the organism's aims and preferences may be conjectured to be the result of natural selection. But once they exist, they are of immense significance in evolution. The idea that environmental conditions do all the sculpturing by carving away what is less fit, and that the organism is passive, is simply false from an evolutionary point of view. Without the organism's struggle for survival there would be no evolution. And this means that it is only the organism's fight for life, its problem solving, its constant search for a better environment, for better living conditions, its active evolution of its own preferences, of its sensitivity, that makes evolution possible. It is the only active agent. It is the environment which is passive. To look at it as hostile, as so many Darwinists do, is mistaking a metaphor for a powerful reality. To think of it as a sculptor, a carver, is to make a god of it. It is always the organism which seeks a better ecological niche, better living conditions, a better life. All else is metaphor. The biologist, observing, experimenting, begins with the stimulus. And he is in danger of believing in the famous couple: stimulus and response. But it is the organism which evolves all sorts of responses to find, interpret, exploit and learn from what we mistakenly look upon as the life-evoking, life-demonstrating stimulus of a (perhaps even passive) response... I wish to end with an important proposition. The difference between the growth of animal knowledge and of human knowledge is just this: human language permits us to formulate our theories outside our skin, exosomatically: this allows us to criticize them. This makes possible the evolution of human reason, of the use of imagination and of criticism in the search for truth. Sum in information philosophy. It is not chauvinistic or parochial to regard it as the highest form of evolved information in the universe. Nor is it naive to regard it as proof of the Idea of Progress and a possible basis for objective value.