Part II of Cheryl Misak’s beautiful intellectual biography of Frank Ramsey is devoted to the young Ramsey’s undergraduate years at Cambridge University. (I reviewed Part I in my previous post.) If there is a common or overarching theme during these formative years in Ramsey’s intellectual life (1920 to 1924), it is Ramsey’s willingness to challenge the most powerful and original ideas of such great and legendary scholars and philosophers as J.M. Keynes, G.E. Moore, Bertrand Russell, and Ludwig Wittgenstein. In this post, I will limit myself to just one such momentous undergraduate episode–Ramsey’s early critique of Keynes’s objective or logical theory of probability.
To appreciate Ramsey’s first foray into probability theory, I must first provide some relevant background. The great Keynes had published his Treatise on Probability in 1921, and in a review of Keynes’s work, none other than Bertrand Russell had called Keynes’s Treatise “the most important work on probability that has appeared for a very long time,” adding that the “book as a whole is one which it is impossible to praise too highly.” (See Russell, 1948 , p. 152.) Why was Keynes’s work so highly praised? Because Keynes had developed a new way of looking at probability, one which allowed for the possibility of probabilistic truth. For Keynes, probability consisted of an objective or logical relation between evidence and hypothesis, or in the words of Misak (2020, p. 113, emphasis added), a relation “between any set of premises and a conclusion in virtue of which, if we know the first, we will be warranted in in accepting the second with some particular degree of belief.”
Ramsey, however, immediately identified two blind spots in Keynes’s conception of probability. (See Ramsey, 1922; see also Misak, 2020, pp. 114-115.) One was Keynes’s admission that not all probabilities are numerical or measurable, especially when the truth values of our underlying premises are in dispute. In that case, when we have no idea whether our premises are true or not, Keynes’s approach does not allow us to measure the probabilities of our conclusions. For Ramsey, by contrast, all probabilities should be measurable. But the other (more deeper) problem with Keynes’s theory was the “objective” nature of his view of probability–the idea that all statements or propositions stand in logical relation to each other. Ramsey denied the existence of these logical relations altogether. Far from being an “objective relation,” the strength or weakness of the relationship between two propositions also depended on psychological factors: on one’s personal experiences and subjective beliefs. In a word, probability was based on experience, not logic. (Sound familiar? If not, check out the quote by the great Oliver Wendell Holmes below.)
Yet, as the old academic saying goes, it takes a theory to beat a theory, and at this stage in his academic career the young Ramsey had yet to develop his own full-fledged theory of probability. But as we shall see in an upcoming blog post, Ramsey would finally get around to doing so in the last half decade of his short life …