Ramsey on Keynes’ Treatise
F. P. Ramsey Mr Keynes on Probability Brit. J. Phil. Sci. 40 (1989), 219-222
Review (1922) of: A Treatise on Probability by J. M. Keynes.
Ramsey here seems to take a commonsense view of probability, and to be concerned about the foundations of the theory. Ramsey argues that:
[T]here is no such probability as the probability that ‘my carpet is blue’ given only that ‘Napoleon was a great general’, and there is therefore no question of assigning a numerical value.
Thus both Ramsey and Keynes seem to agree that some probabilities can be assigned a numerical value. The difference seems to be that Keynes considers a broader, problematic, range of cases and thus deals with things that Ramsey does not consider to be ‘real’ probabilities.
Ramsey supposes that:
[W]e are concerned with the relation which actually holds between two propositions; the faculty of perceiving this relation, accurately or otherwise, we call insight, perfect or imperfect.
Thus, in distinction to his later view, he seems to suppose that there is some actual ‘objective’ probability relation that we are trying to assess. Keynes is seeking a much broader theory.
In considering Keynes notion that probability is relative to evidence, Ramsey notes:
In everything, it might be urged, owing to the possibility that there is evidence to which we have no access, we are only justified in saying not ‘p’ but ‘We have reason to suppose p’. The logical conclusion of this view is that we are not justified in saying anything at all; for our evidence about human reason might also be fragmentary. We cannot therefore reasonably say ‘We have reason to suppose the probability is a’, but only ‘We have reason to suppose that we have reason to suppose the probability is a’, and so on ad Infinitum—on the lines of a celebrated argument in Dr Moore’s Ethics.
It seems to me that when I say ‘p’ I usually mean that ‘I have an adequate reason, in the context, to say p’, which would normally imply that I ‘supposed’ p. I also habitually interpret other’s utterances in this way. This thinking of mine has its ancestry in Keynes, so I tend to agree with Ramsey that this is a reasonable interpretation of Keynes’ Treatise. Thus a claim about probabilities would be based on experience and only ‘good’ for ‘the foreseeable future’, for as long as the context lasted.
Ramsey’s objection to this seems to me largely technical and entirely unappealing. It is quite true that my appreciation of ‘human reasoning’ is empirical and hence my notion of ‘good reasoning’ is uncertain. But I can make my own ‘best effort in the circumstances’ and rest on that without recursion. Certainly I tend not to suppose that others are reasoning to any particular standard, but only that their mode reasoning is ‘reasonable’ with regard to their culture and the circumstances.
With Keynes and Good in mind, we might be more specific: when we say ‘p’ we at least mean that p appears to us to be consistent with what we know and that there are no ‘significantly different’ alternatives that we have thought of and which seem to us reasonable to take account of. For example, if – in the morning – my partner says that our car has a full tank of fuel, I do not assume that she has kept guard over it all night to prevent anyone from syphoning off the fuel. Thus Ramsey’s objection seems not to apply in practice. Ramsey does make some other valid technical points, but they do not affect ‘the big picture’ of the Treatise, which is that probability cannot always be treated as a number, that there are some ‘work-arounds’, and even if not it can pay to be aware of when one is in a complex situation.
- IT’S OFFICIAL: Keynes Was Right (businessinsider.com)