A World of Propensities – Karl Popper

Two New Views of Causality and the Evolution of the World

1. The Propensity Interpretation of Probability

We begin with the opening lecture, in which Popper sets out to replace two old metaphysical doctrines—determinism and indeterminism as traditionally conceived—with a new view of the universe built on the concept of propensities. Popper locates his argument within the long history of the interpretation of probability, noting that he first introduced the propensity interpretation in 1957, but that he now intends to take it much further.

Popper presents the problem directly. He states that the classical interpretation of probability, whether the subjective or the frequency interpretation, fails to account for the singular character of probabilistic events. The frequency interpretation, for example, treats probability as the limit of relative frequency in a long sequence of repeated events. But this leaves no room for the probability of a single event—the chance that this particular coin will land heads on this particular toss. Popper argues that we need an interpretation that applies to the single case, and that this is precisely what the propensity interpretation provides.

He writes:

“The propensity interpretation regards probability as a property of the experimental arrangement; that is to say, of the conditions of the experiment, and as a measure of the strength of the propensity, or tendency, of a typical experimental arrangement to give rise to a certain result.”

This is the key move. Probability is not in our heads (subjective), nor is it an abstract feature of long runs (frequency). It is a real, physical property of the situation itself—the setup, the conditions, the entire concrete experimental arrangement. The coin has a certain weighted tendency to fall heads, and this tendency is as real as its mass or its shape. Popper insists that this is a metaphysical view: the world contains real tendencies, not merely regularities. These tendencies are not reducible to occurrences; they are the causal powers that produce occurrences.

Popper clarifies what this means for causality. The old “billiard ball” determinism imagined that every event is completely caused by prior events with no looseness or wiggle room. The propensity view does not abolish causality; it reinterprets it. Causes are propensities: they tend to produce certain effects with certain weights. Some propensities are very strong (near-certainty), and some are weak. All physical processes involve such propensities, not iron necessities. The world is therefore not a deterministic machine but a field of propensities—a world of real possibilities that are partly actualized as events unfold.

2. The Propensity Interpretation and Quantum Theory

Popper now turns to quantum mechanics, which he considers the domain where his interpretation proves its worth. He locates himself in a long debate: the Copenhagen interpretation of Bohr and Heisenberg treats quantum probabilities as fundamentally unknowable or as merely mathematical, while Einstein held that “God does not play dice.” Popper sides with neither fully. He agrees with Einstein that reality is objective, but he disagrees that quantum indeterminacy is merely a veil over hidden variables.

He presents the core quantum phenomenon: the two-slit experiment. A single electron is fired at a screen with two slits. The electron appears at a definite point on the detector behind, yet the pattern that builds up over many firings shows interference fringes—as if the electron went through both slits. Popper argues that the propensity interpretation makes sense of this. The experimental arrangement—the slits, the screen, the detector—confers on the electron a propensity to land at various points. Each point has a certain propensity weight. The electron’s actual landing is the realization of one of these propensities. The wave function, in this view, is not a physical wave but a mathematical description of the propensities inherent in the experimental situation.

Popper writes: “The wave function is a mathematical expression of the propensities of the experimental arrangement. It does not describe a real state of the electron, but rather the tendencies of the experimental arrangement to produce a certain result.” This avoids the paradoxes of wave-particle duality, because the electron is always a particle; what is wave-like is the field of propensities that determines the probabilities of its behavior.

He clarifies why this matters within his overall project. Quantum theory has been distorted by positivist and instrumentalist philosophies that deny we can speak of what is “really” there. Propensity realism restores ontology: the world of atoms and subatomic particles is a world of real tendencies, not just mathematical fictions. The indeterminacy of quantum events is not a sign of incompleteness but a fundamental feature of reality. The universe, at its most basic level, is propensity-laden, not deterministic.

3. Objective Probability and the Problem of Determinism

Popper now broadens his argument into a metaphysical critique of determinism. He locates his target: the “scientific determinism” that claims all events are predetermined by prior physical conditions and laws, making the future as fixed as the past. This view, he argues, is not a consequence of science but a philosophical prejudice inherited from Newtonian mechanics.

He presents a key passage:

“The doctrine that all future events are predetermined—that they are fixed in advance—is not a doctrine that can be proved or refuted by science. It is a metaphysical doctrine. But it is a metaphysical doctrine that has very real consequences for our view of human freedom and creativity.”

Popper then contrasts two worlds. In a deterministic world, everything that will happen is already implicit in the present state; there is no genuine novelty. The future is a rearrangement of the past. In a propensity world, by contrast, the future is open: new propensities can emerge, and the actual course of events brings into existence what was only tendential before. This openness is not chaos—propensities are real causal tendencies—but it is genuine indeterminism.

He illustrates with the example of the growing organism. A fertilized egg has propensities to develop into a particular kind of adult organism, but these propensities are not fixed. Environmental conditions, chance mutations, and developmental interactions can shift the weights. The outcome is not inevitable; it is a product of the propensities at work. Popper stresses that this view is compatible with science: it does not deny regularities or laws, but it reinterprets laws as descriptions of propensities rather than as iron rails that force events along a single track.

The clarification here is crucial: Popper is not simply defending randomness. He is defending real possibility. A universe of propensities is one in which possibilities have degrees of reality—some are more probable than others, but none are absolutely fixed until the event occurs. This makes room for emergent novelty in evolution, in history, and in human action.

4. The Evolutionary Epistemology of Propensities

Popper now extends his view into the realm of knowledge and evolution. He locates his argument in the context of his earlier work on evolutionary epistemology, particularly in *Objective Knowledge*. The key insight is that life itself arises through the reality of propensities: the physical world, given its propensity structure, tends to produce certain kinds of chemical and biological organization.

He presents a striking claim: the universe is not just a stage for propensities; it is itself a product of propensities. The initial conditions of the Big Bang conferred on the universe certain propensities for the development of heavy elements, stars, planets, and eventually life. These propensities were not certainties—they were real tendencies that, under the right conditions, were actualized. Popper writes: “The universe, at its beginning, had a propensity to produce life. And life, once it emerged, had a propensity to produce consciousness. And consciousness, once it emerged, had a propensity to produce works of art, of science, of morality.”

This is not teleology or design. It is a claim about the causal structure of reality: given the laws and initial conditions, some outcomes are more probable than others, and these probabilities are real features of the world. The evolution of life is not a matter of blind chance alone; it is a matter of propensities realizing themselves through interaction with the environment. Natural selection works on propensities: organisms with certain propensities survive and reproduce, while others do not. But the propensities themselves are not merely statistical; they are built into the organism’s structure and its relation to its environment.

Popper clarifies the epistemological implication. Knowledge, too, grows through a process of conjecture and refutation, which mirrors the trial-and-error structure of evolution. Our theories are propensities: they have a tendency to approximate truth, but they are never certain. The growth of knowledge is the actualization of certain epistemic propensities that the universe, through conscious beings, has developed. This is a profound unification: the same metaphysical framework—propensities as real causal tendencies—applies across physics, biology, and epistemology.

5. Propensities and the Arrow of Time

We move now to a section where Popper confronts one of the most persistent puzzles in physics: the nature of time’s directionality. Here he locates the work within the broader project of reconciling probability with temporal asymmetry.

Popper begins by stating a fundamental observation: “The arrow of time is closely connected with the increase of entropy.” This is the well-known Second Law of Thermodynamics, which states that in an isolated system, disorder (entropy) tends to increase over time. The puzzle is that the fundamental laws of physics are time-symmetric—they work the same forward and backward—yet we observe a clear direction from past to future. Popper argues that the propensity interpretation can resolve this apparent contradiction.

He presents the standard statistical interpretation of entropy, associated with Boltzmann, which explains the Second Law as a probabilistic tendency: systems move from less probable to more probable configurations. In the classical view, this is a matter of ignorance—we simply do not know the exact microstates, so we describe the system statistically. But Popper insists that this is not enough. The tendency toward increasing entropy must be understood as a real propensity of the physical world, not merely a reflection of our incomplete knowledge.

“The tendency of physical systems to increase their entropy is not a consequence of our lack of knowledge; it is a real physical propensity.”

Popper develops this argument by considering what happens when we reverse the direction of time in our descriptions. If we take a system in a low-entropy state and ask about its past, the statistical interpretation seems to imply that it is equally probable to have come from a higher-entropy state. But that would violate the Second Law, which tells us that entropy was even lower in the past. The standard response is to posit a special initial condition—the universe began in a very low-entropy state, and we observe the increase from that starting point. Popper finds this ad hoc. Instead, he proposes that the propensity toward increasing entropy is a fundamental feature of the universe’s dynamics, not a boundary condition.

The clarification here is that Popper is offering a metaphysical rather than a purely statistical solution. The arrow of time is real because propensities themselves are time-asymmetric: a system has a propensity to move toward equilibrium, not away from it. This propensity is encoded in the physical structure of the system, just as a six-sided die has a propensity to land on each face. The increase of entropy is not a mysterious fact about the universe’s initial conditions; it is an expression of the real, causally efficacious tendencies that govern all physical processes. By grounding the arrow of time in propensities, Popper aims to make the Second Law a necessary feature of a world that operates through real tendencies, not an accidental consequence of a statistical fluctuation.

6. Propensities and the Mind-Body Problem

The next major movement in the work takes up a question that might seem far removed from physics: the relationship between mind and matter. Popper positions this section as an extension of his propensity framework into the realm of consciousness and mental causation.

Popper begins by acknowledging the “mind-body problem” as one of the most difficult in philosophy. The traditional debate is between materialism (mind is reducible to matter), idealism (matter is reducible to mind), and dualism (mind and matter are distinct substances). Popper rejects all three in their classical forms. He proposes instead that both mind and matter are expressions of a deeper reality: the propensity structure of the universe. The mind is not a separate substance but a “propensity for interaction” — a set of real tendencies that emerge from the physical organization of the brain and its environment.

He develops this idea by examining the phenomenon of conscious experience. Consciousness, Popper argues, is not a passive epiphenomenon—a mere byproduct of physical processes without causal power. Rather, it is a real causal factor in the world. When we make a decision, our conscious deliberation alters the distribution of propensities for future actions. This is not a violation of physical law but an expression of the way propensities operate at higher levels of organization. The brain, as a physical system, has propensities to produce certain thoughts and actions; consciousness is the subjective experience of those propensities being weighed and actualized.

“The mind is not a thing; it is a process—a propensity to interact with the world in certain ways.”

Popper clarifies the ontological status of this claim. He is not advocating a return to Cartesian dualism, where mind and matter are separate substances. Instead, he proposes a pluralistic ontology: the world consists of many levels of reality, each with its own propensities. The physical level has propensities for motion and change; the biological level has propensities for survival and reproduction; the mental level has propensities for thought, feeling, and decision. These levels are not reducible to one another, nor are they independent; they interact through the shared medium of propensities. The mind-body problem is thus dissolved: there is no gap between mind and matter because both are manifestations of the same underlying tendency-structure of reality.

The significance within the work is that Popper is extending his metaphysical framework from the physical sciences to the human sciences. If propensities are real, then mental events are real causal factors, and we are not forced into a crude materialism that denies the efficacy of consciousness. At the same time, he avoids the problems of traditional dualism by grounding mental propensities in the physical organization of the brain. The mind is a real emergent propensity, not a ghost in the machine.

7. Propensities and Human Freedom

This section directly confronts the most profound implication of the propensity view: what it means for human agency and moral responsibility. Popper frames this as the culmination of his metaphysical system—the place where physics, biology, and ethics converge.

Popper begins by stating the classical problem of free will: if the universe is deterministic, then every human action is the necessary consequence of prior causes, and freedom is an illusion. If the universe is purely random, as some interpretations of quantum mechanics suggest, then actions are arbitrary and not genuinely free either. The propensity interpretation offers a third way: the world is neither deterministic nor random, but propensity-laden. Human actions are the actualization of propensities that are shaped by our character, our reasoning, and our environment, but they are not predetermined with certainty.

He develops this by analyzing the nature of decision-making. When we deliberate, we are not merely calculating the outcome of a deterministic algorithm. We are weighing propensities—considering the tendencies of various courses of action to lead to certain results. Our deliberations themselves alter the propensities: by thinking, we change the weight of different possibilities. Popper uses the example of a chess player: the player considers possible moves, each of which has a propensity to lead to a win or a loss. The decision is not forced by prior conditions; it is a creative act that actualizes one propensity among many, influenced by but not determined by the player’s knowledge and skill.

“We are not forced by the past; we are guided by propensities—tendencies that are real but not determining.”

Popper clarifies the ethical dimension. If propensities are real, then we can be held responsible for our actions because we genuinely could have done otherwise. The actualization of a propensity is not a random event; it is a choice that reflects our character and reasoning. At the same time, our character itself is a product of prior propensities—genetic, environmental, and cultural—so we are not absolutely free in the sense of being uncaused. But this is not a contradiction: propensities allow for degrees of freedom. A person with a strong propensity for honesty is not forced to be honest; the propensity is a tendency that can be overridden by deliberation or by other propensities. Freedom consists in the ability to weigh and choose among competing propensities, not in the absence of any causal influence.

The deep point Popper makes is that human freedom is a real feature of a propensity-laden world. Determinism denies it, and indeterminism (pure randomness) makes it meaningless. Propensities preserve freedom because they are tendencies, not certainties. We are not playthings of fate, nor are we random generators; we are creatures whose actions manifest real propensities that we can—through reason, reflection, and effort—modify and redirect.

8. Propensities and the Problem of Induction

We are now in a section where Popper applies the propensity interpretation to one of the deepest problems in philosophy: how we can justify our belief that the future will resemble the past. This section reexamines Hume’s problem of induction through the lens of propensities.

Popper begins by stating the problem in its classical form. David Hume argued that we have no rational justification for inductive inferences—the inference from past regularities to future ones. If all swans observed so far have been white, we have no logical guarantee that the next swan will be white. Hume concluded that our belief in induction is a custom or habit, not a rational deduction. Popper agrees with Hume’s negative conclusion about logical justification but argues that the propensity interpretation offers a new way to understand what we are actually doing when we make inductive predictions.

“I hold that induction is a myth. There is no such thing as induction in the sense of a method that leads from observation statements to theories. All that exists is the method of trial and error: the method of hypothesis and refutation.”

Popper then clarifies the connection to propensities. When we observe a regular pattern in nature—say, that coins land heads about half the time—we are not observing a “law” that forces the next toss to produce a certain outcome. Rather, we are estimating a propensity that characterizes the experimental arrangement. The propensity is not in the past observations but in the physical situation itself. Past frequencies are merely evidence that helps us estimate the propensity; they do not constitute the propensity itself. This shifts the epistemological problem: we are not trying to justify a logical inference from past to future, but rather trying to estimate an objective propensity that exists now, in the present experimental setup.

The key move is this: Popper argues that the problem of induction dissolves when we realize that scientific theories are not inferred from observations but are conjectures about propensities. We propose that a certain physical system has a certain propensity distribution—say, a radioactive atom has a 50% propensity to decay within its half-life. We then test this conjecture by experiment. The experiments give us frequencies, which we use to check whether our estimated propensity is accurate. But the connection between past frequencies and future events is not logical; it is physical. The same experimental arrangement that produced the past frequencies continues to exist, so the same propensities continue to operate.

Popper acknowledges that this seems circular: we use past frequencies to estimate propensities, but propensities are supposed to explain those frequencies. He replies that this circularity is not vicious because propensities are independently testable. We can design new experiments that probe the same physical system, and if our estimated propensity is correct, the new frequencies will match. The process is one of conjecture and refutation: we guess the propensity, we test it, and if it survives testing, we provisionally accept it. No induction is ever justified; we simply have better and better approximations of objective propensities.

This reframing matters because it avoids the classic problem of induction without resorting to skepticism or to Hume’s psychological explanation. Popper does not claim that we can prove the future will resemble the past; he claims that we do not need to. What we need is a realist interpretation of probability that allows us to treat probability statements as true or false descriptions of the world. The propensity interpretation provides that: a claim like “this coin has a propensity of 0.5 to land heads” is a physical hypothesis, not a summary of past observations. It can be false, and we can discover its falsity by experiment. Induction is not a method of discovery; it is a myth. What we actually do is conjecture propensities and test them.

Propensities and the Problem of Induction

We are in the section where Popper applies his propensity interpretation to the oldest and most persistent problem in his philosophy: the problem of induction. This section presents the propensity theory as the key that finally unlocks the puzzle that Hume posed and that Popper has spent his career addressing.

Popper begins by restating Hume’s problem in its starkest form. Hume showed that we cannot justify induction by logic, because induction assumes the very uniformity of nature that it is supposed to prove. Nor can we justify it by experience, because any appeal to past successful inductions is itself an inductive argument. Hume concluded that induction is not rational but merely a habit or custom of the mind. Popper agrees that induction has no logical justification, but he rejects Hume’s psychological explanation. There is, Popper argues, a third possibility: that induction is neither logically justified nor merely habitual, but that it is replaced entirely by a different method — the method of conjecture and refutation.

Now, with the propensity interpretation, Popper can deepen this replacement. He writes:

“The propensity interpretation allows us to see that probability statements are not about the frequency of events in actual finite sequences, but about the physical propensities of the experimental arrangement — the entire repeatable objective situation.”

This is the crucial move. Hume’s problem arises because we try to move from observed frequencies to unobserved frequencies. But if probability statements are really about propensities — physical properties of the world that exist whether or not we observe them — then the whole problem takes a different shape. When we say “this coin has a probability of 0.5 of landing heads,” we are not making a prediction about a finite sequence of tosses. We are making a claim about the weighted possibilities inherent in the coin and the tossing arrangement itself. This claim is a conjecture — a hypothesis that can be tested, but that never requires inductive inference to be formulated.

The key distinction Popper draws is between statistical statements about finite sequences and propensity statements about objective single-case probabilities. A statistical statement like “this coin landed heads 50 times out of 100” is a report of what has happened; it can be verified or falsified by direct observation. But a propensity statement like “this coin has a propensity of 0.5 to land heads” is a physical hypothesis that goes beyond any finite set of observations. It is not derived from observations; it is conjectured to explain them and tested against them. This is exactly the same logic that Popper applies to all scientific hypotheses. The propensity theory does not solve the problem of induction by justifying it; it dissolves the problem by showing that science never needed induction in the first place.

Popper then addresses what he calls the main argument against induction: the argument from the infinite variety of possible worlds. If the world were different — if quantum mechanics were deterministic, if the laws of nature changed every Tuesday — then past observations would give no guidance. But the fact that the world has been regular so far does not prove it will remain regular. The propensity interpretation shows why this argument is decisive: if probabilities are objective propensities of the experimental situation, then any change in the situation changes the propensities. We cannot infer from the propensities of past experiments to the propensities of future experiments unless we assume that the relevant conditions remain the same. But this assumption is itself a conjecture, not a fact established by induction.

The connection between propensities and the problem of induction becomes clearest when Popper discusses what he calls the propensity to produce frequencies. An objective propensity of 0.5 does not guarantee that in any finite sequence the frequency will be exactly 0.5. It guarantees only that the sequence will tend towards that frequency as the number of trials increases. This tendency is itself a physical fact about the world — it is what Popper calls a propensity of the second order. The first-order propensity is the tendency of a single event to occur; the second-order propensity is the tendency of a sequence of events to converge on a certain frequency. Both are real, objective properties of the experimental arrangement.

What this means for induction is that we do not need to assume that the future will resemble the past. We need only to conjecture that certain experimental arrangements have certain propensities, and then test those conjectures. Induction is not a method for discovering propensities; it is a myth that disguises what we actually do. What we actually do is propose hypotheses about propensities, deduce predictions from them, and test those predictions against experience. The propensity interpretation thus completes Popper’s anti-inductivist epistemology by providing a realist semantic for probability statements — a way of treating them as true or false descriptions of the world, not as summaries of past observations or as expressions of our ignorance.

Popper concludes this section by returning to the theme of realism that runs through the entire work. The problem of induction, he argues, is a problem only for those who think that knowledge must be based on evidence. But if knowledge is always conjectural, always tentative, always subject to revision, then induction is unnecessary. The propensity interpretation gives us a way of thinking about probability that is consistent with this fallibilist epistemology. Probability statements are not inductive generalizations; they are physical hypotheses. And like all physical hypotheses, they can be tested, falsified, and replaced by better ones. This is the real solution to Hume’s problem — not a justification of induction, but an explanation of why science never needed it.

The Open Universe and the Propensity World

In the final substantial section of the work, Popper draws together the threads of his argument to present a comprehensive picture of what he calls the propensity world or the open universe. This is the culmination of the entire lecture: a vision of reality as fundamentally indeterministic, creative, and weighted toward possibilities.

Popper begins by contrasting the propensity world with the clockwork universe of classical physics. In the classical view, the world is like a perfect mechanism: every event is determined by prior events, and the future is contained in the present as surely as the present is contained in the past. There is no genuine novelty, no creativity, no openness. Everything that will happen is already implicit in the initial conditions. Against this, Popper sets the propensity world: a world in which the future is genuinely open, in which possibilities are real, and in which new possibilities emerge as the universe evolves.

He writes:

“The world of propensities is not a world of chance events; it is a world of real possibilities — a world in which the future is open and in which possibilities are weighted, some more likely than others.”

This is the central claim. The propensity world is not a world of pure randomness, where anything can happen with equal probability. It is a world of weighted possibilities, where some outcomes are more likely than others because the propensity structure of reality is itself structured and hierarchical. The propensities are not uniform; they are shaped by the history of the universe, by the laws of physics, and by the specific conditions of each situation.

Popper then introduces the concept of emergent propensities — new possibilities that arise as the universe becomes more complex. In the early universe, he suggests, there were only a few simple propensities: the propensities of elementary particles to interact, to form atoms, to radiate energy. But as atoms formed stars, and stars formed planets, and planets formed living organisms, new propensities emerged that did not exist before. The propensity of a biological organism to reproduce, to mutate, to evolve — these are not reducible to the propensities of elementary particles. They are emergent properties of complex systems, and they become real forces in the world, shaping the course of evolution.

This is where the evolutionary epistemology of the earlier sections connects to the ontology of propensities. Popper argues that living organisms are themselves propensity-exploring systems — they test the possibilities of their environment, discover new propensities, and adapt to them. The evolution of life is not a process of random variation and selection alone; it is a process in which organisms actively explore the space of propensities and bring new possibilities into actuality. The evolution of the eye, for example, is not just the result of random mutations; it is the result of organisms exploring the propensity of light to carry information, and the propensity of certain chemical structures to detect that information.

The concept of emergent propensities leads Popper to a radical conclusion about the nature of time. If new propensities emerge as the universe evolves, then the future is not simply unknown; it is not yet determined. The future is genuinely open, not just in the sense that we do not know it, but in the sense that it does not yet exist as a determinate reality. This is what Popper calls the open universe: a universe in which the future is created as it goes along, not merely unfolded from a pre-existing blueprint.

Popper then addresses a potential objection: if the future is open, how can we have any knowledge at all? If propensities change and evolve, can we make any predictions? His answer is that predictions are possible precisely because propensities are relatively stable in the short term. The propensities of a coin toss do not change from moment to moment; the propensities of the weather do not change minute by minute. This relative stability allows us to make conjectures and test them. But the stability is not absolute; propensities can shift, and when they do, our predictions fail. This is not a failure of science; it is a discovery of new propensities.

The final pages of the work return to the theme of human freedom that appeared in the earlier sections. Popper argues that the propensity world provides the metaphysical foundation for genuine human agency. In a deterministic universe, our choices are illusions; they are the necessary consequences of prior causes. In an indeterministic universe, our choices can be real — not random, not determined, but shaped by our values, our knowledge, and our character. The propensity interpretation gives us a way of understanding how this works: our choices are not determined by prior causes, but they are weighted by our propensities. We have propensities to act in certain ways, but these propensities are not absolute; they can be changed by reflection, by learning, by effort.

Popper concludes with a vision of the universe as a creative process — a process in which new possibilities are constantly being realized, and in which the future is not a pre-written script but a story that is being written as it unfolds. The propensity world is a world of risk and opportunity, of uncertainty and hope. It is a world in which we can make a difference, because our actions are not predetermined; they are real choices that shape the future.

This is the final image of *A World of Propensities*: a universe in which possibilities are real, in which time is genuinely open, in which novelty emerges, and in which human freedom is possible. The propensity interpretation of probability, which began as a technical solution to a problem in the philosophy of physics, ends as a comprehensive metaphysics — a vision of reality as dynamic, creative, and alive with possibility.