Theory mainly associated with Karl Raimund Popper (1902-1994), though it goes back to Charles Sanders Peirce (1839-1914). Popper introduces it to replace the frequency theory of probability in view of an objection he brings to that.
Probabilities are propensities, not of objects under study but of the experimental arrangements which we keep constant during repeated experiments. Though not directly observable, Popper claims that propensities are no more mysterious than forces or fields, and that their existence can be falsified (see falsificationism). This, however, may be doubted, if the run of relevant events is potentially infinity; we may be forced to say that probably the propensity exists, or (if the propensity is simply identified with the evidence for it) that it is rational to act on it because probably it will continue.
C S Peirce, Collected Papers, 2 (1932), 404-14;
K R Popper, 'The Propensity Interpretation of Probability', British Journal for Philosophy of Science (1959)