Philosophical dialogue is a curious activity. Arguments are expected to be rigorous, but no demand is made that there must be evidence for the premisses. Terminology is expected to be precise, but its appropriateness to the subject matter under discussion can be left unexplored. Officially, nothing is conceded; but, in fact, a great deal is taken for granted. Ad argumentum mingles indiscriminately with ad hominem; and, above all, the evidential warrant for one’s philosophical claims is, like the topics of sex and religion to the less enlightened, one of those delicate issues never to be discussed in mixed company.

Such conventions as these that are associated with contemporary philosophical exchange make it difficult to have a balanced discussion about Progress and Its Problems. That work is, in the first instance, a descriptive model of theory change in science. It purports to establish what sorts of factors have in fact influenced scientific decision-making.

By “Propensities” I mean the kind of probabilities that figure in laws of nature. Propensities might be (i) relative frequencies, finite or long run, de facto or modalized, or (ii) reflections of our epistemic probabilities or (iii) sui generus theoretical notions. I believe that the whole family of relative frequency proposals (i) are inadequate. As an alternative I wish to suggest (ii) an epistemic account of propensities and of nomic force in general, in the spirit of Hume, Mill, DeFinetti, Ayer, Suppes and Jeffrey. Whether accounts of the third kind differ in substance or in name only from the sort of account that I am proposing is a nice question, to which. I will devote some brief closing remarks.

In this paper I consider the question of the reality of chance. This is not what divides contemporary probabilists into the objective and subjective schools. That division is accomplished by the question whether there are objective grounds for the correctness of probability judgments. The subjectivists say that there need not be such grounds, and that probability judgments thus need not be empirically meaningful in the verificationist sense, or perhaps that they are not judgments at all, but rather expressions of attitude. (See [11], pp. 156-198). The objectivists say that the truth conditions for probability judgments are such as the frequencies of traits in populations or the existence of propensities. Objectivists and subjectivists are, however, agreed that there are no objective chances. (See [2], pp. 141, 142; and [12], §16).

There is a Humean argument against the reality of chance which depends mainly upon the premise that we have no experience of it.