“A World of Propensities”, a slender 51 pages, was published in 1990. It contains two lectures that Popper delivered in his 87th and 88 years, A World of Propensities: Two New Views of Causality and Towards an Evolutionary Theory of Knowledge.
The book “All Life Is Problem Solving” (1999) also contains the essay, “Towards an Evolutionary Theory of Knowledge”. This lecture is a classic of how to approach the topic of knowledge without getting tangled in the regress of chasing definitions. Open any dictionary on the word “knowledge”, you will find all sorts of circularity and assumptions that knowledge is primarily empirically derived. Popper’s starting point is a very simple proposition that animals can know something: that they can have knowledge. He elegantly proposes that knowledge is linked to expectations. These expectations express theories of reality. We as with all living things have propensities to guess reality based on largely unconscious hypotheses which both logically and psychologically precede observation. Popper’s association of knowledge with expectation, or guessing, is a breakthrough in clarity. Animals and plants carry what can be defined as unconscious guesses or theories, namely initially their genes and other molecular and physiological codes.
In “A World of Propensities” Popper recounts his debt to Alfred Tarski and his view of truth as a correspondence of a statement with the facts. It is a theory of objective truth that requires us to distinguish clearly between truth and certainty. Popper also recounts his rejection of probabilistic induction and how shocked he was when Carnap followed the probability of hypotheses line in “The Logical Foundations of Probability”(1950). “I felt as a father must feel whose son has joined the Moonies; though of course they did not exist in those days.”
Popper’s propensity theory is an objective interpretation of the theory of probability. Propensities, it is assumed, are not mere possibilities but are physical realities. They are as real as fields of forces and vice versa. Propensities in physics are properties of the whole physical situation. Propensities, like Newtonian attractive forces, are invisible, and, like them, they can act: they are actual, they are real. However, neither our physical world nor our physical theories are deterministic, even though of course many possibilities are excluded by the laws of nature and of probability: there are many zero propensities. The future is open. It is especially obvious in the evolution of life that the future was always open.
Accidents and preferences.
“The theory of motives determining our actions, and the theory that these motives in their turn are motivated or caused or determined by earlier motives etc., seems, indeed, to be motivated – motivated by the wish to establish the ideology of determinism in human concerns. But with the introduction of propensities, the ideology of determinism evaporates. Past situations, whether physical or psychological or mixed, do not determine the future situation. Rather, they determine changing propensities that influence future situations without determining them in a unique way.
And all our experiences – including our wishes and our efforts – may contribute to the propensities, sometimes more and sometimes less, as the case may be. (In spite of the instability of the weather, my wishes do not contribute to ‘sunshine tomorrow’. But they can contribute a lot to my catching the flight from London to San Francisco.)
In all these cases the propensity theory allows us to work with an objective theory of probability. Quite apart from the fact that we do not know the future, the future is objectively not fixed. The future is open: objectively open. Only the past is fixed; it has been actualized and so it has gone. The present can be described as the continuing process of the actualization of propensities; or, more metaphorically, of the freezing or the crystallization of propensities. While the propensities actualize or realize themselves, they are continuing processes. When they have realized themselves, then they are no longer real processes. They freeze and so become past – and unreal. Changing propensities are objective processes, and they have nothing to do with our lack of knowledge; even though our lack of knowledge is, of course, very great, and even though a particular lapse may, of course, be an important part of the changing situation.”