Chapter 8 of Donald Gillies’ Philosophical Theories of Probability (2000) deals with intersubjective and pluralist views of probability.
Gillies (2000: 169) argues that there is an intermediate class of probability called intersubjective probability that lies between fully subjective and objective probability.
Furthermore, Gillies makes the case for a “pluralist” view of probability, a development of the two-concept view of probability by Ramsey and Carnap (Gillies 2000: 180–181). In Gillies’ “pluralist” theory of probability, probabilities can be conceptualised on a spectrum or continuum from the fully objective to fully subjective, and intermediate types of probability, as in the diagram below (which needs to be opened in a new window to be seen properly).
A fully objective probability is one that is found in natural phenomena or the material world and is independent of human beings, such as, for example, the probability of radioactive decay of atoms (Gillies 2000: 180).
An artefactual probability is objective in the sense that it is part of the material world, but is at the same time the result of human interaction with nature (Gillies 2000: 179). These include probabilities of games of chance, probabilities of repeatable scientific experiments, and certain statistical probabilities. If such probabilities can be established though stable long-run observed relative frequencies in trials or experiments, then they can be said to be objective (Gillies 2000: 180).
Between artefactual and intersubjective probabilities, Gillies sees some borderline cases such as in medicine or population studies (Gillies 2000: 180, 194).
An intersubjective probability is a measure of the degree of belief of a social group where a consensus has formed (Gillies 2000: 179). An intersubjective probability may be utterly subjective, or subjective but based on an underlying objective probability. If a group of people, though shared beliefs, can agree on a common betting quotient about some probability, then this can be seen as an intersubjective or consensus probability (Gillies 2000: 171).
A fully subjective probability is one of an individual and is a mere measure of a degree of belief, which has a non-objective status (Gillies 2000: 179).
Fully objective and artefactual probabilities are objective and properly described by the propensity theory of probability, but intersubjective and fully subjective probabilities are epistemological, and often such subjective probabilities have no real nor quantifiable objective “propensity” probabilities at all (Gillies 2000: 180–185).
“Gillies on Probability and Uncertainty in Keynes’ Economic Thinking,” July 6, 2014.
“Interpretations of Probability,” Stanford Encyclopedia of Philosophy, 2002 (rev. 2011)
Gillies, D. A. 2000. Philosophical Theories of Probability. Routledge, London.