The key here seems to be a vague and embryonic outline of some of the ideas of behavioural finance. How do we deal with uncertainty when there is no basis to make an estimate of probabilities? What short-cuts can the mind use to deal with these issues? One of the things that Keynes suggests here and the The General Theory is the idea that we assume that things will be rather similar to how they have been in the past. This is probably a reasonable starting point. It is conservatism. Another thing that Keynes suggests is that we look to the opinion of others. Here we get the basis for a social construction of belief.