Essay: The Propensity Interpretation of Probability

Overview
Karl Popper develops the propensity interpretation of probability as an objective account that treats probabilities as physical properties of experimental setups. Probability is not reducible to long-run frequencies or to subjective degrees of belief; instead it is a dispositional feature of a situation that predisposes certain outcomes. The interpretation aims to make sense of single-case probabilities and to provide a realist grounding for probabilistic statements used in science.

The Propensity Concept
A propensity is a tendency or disposition of a given experimental arrangement to produce a particular outcome. That disposition is a physical attribute of the whole setup, including initial conditions and relevant background circumstances. Probabilities are the quantitative measures of these dispositions: they express how strongly a configuration tends to produce one outcome rather than another, and they are objective in the sense that they are properties of the physical world rather than matters of subjective judgment.

Relation to Frequencies and Subjectivism
Popper rejects the idea that probabilities are only descriptive statistics of infinite sequences of trials, because many scientific and everyday statements concern single cases or finite sequences. Frequencies can confirm or estimate propensities but cannot exhaust their conceptual content, since a single-case propensity can exist even before repeated trials are performed. Likewise, propensities differ from subjective degrees of belief because they are supposed to be independent of an agent's knowledge; they are causal or dispositional features that would produce certain relative frequencies if the same experimental conditions were repeated.

Key Arguments and Examples
Popper uses examples like biased coins, radioactive decay, and controlled experiments to illustrate how propensities function. A biased coin has a propensity to land heads with a certain numerical strength; radioactive nuclei have propensities to decay within specified intervals. These propensities are not mere shorthand for observed frequencies but explanatory factors that account for why particular frequencies would be obtained under repetition. Popper argues that propensities obey probability axioms and that conditional probabilities can be understood as propensities under specified experimental conditions, thereby preserving the formal apparatus of probability while changing its metaphysical interpretation.

Objections and Responses
Several challenges confront the propensity view, including questions about how to define the relevant "whole experimental arrangement" and how to assign precise numerical values to propensities. Critics worry about circularity if propensities are defined by reference to expected frequencies, and about whether propensities can be empirically verified beyond appeal to frequencies. Popper responds by insisting on a causal and objective reading: propensities are testable insofar as they make probabilistic predictions about what frequencies will be observed under repeated applications of essentially the same conditions. The interpretation reframes verification as an interaction between theory and repeatable experimental setups rather than as mere tallying of occurrences.

Legacy and Significance
The propensity interpretation provided a fresh, realist option in debates about probability and influenced later discussions in the philosophy of science and foundations of quantum mechanics. It appealed to those seeking an account of single-case probabilities and to physicists grappling with indeterministic laws. While not universally accepted, the proposal stimulated further work on dispositional properties, objective chances, and the role of probability in causal explanations, keeping the notion of probability tethered to physical practice and experimental design rather than to purely mathematical or epistemic frameworks.