Keynes proposes that probability is a relation that a proposition only has a probability in relation to another. There needs to be some reference point. He compares this to saying something is a certain distance away. You need to specify, distance from what.
Keynes chooses as a reference point our own corpus of knowledge, the set of propositions that we hold.
There is nothing novel in the supposition that the probability of a theory turns upon the evidence by which it is supported
On a definition of the probability of an argument, Keynes says that its a new logical construct, this probability relation that seems to equate to degrees of belief in a proposition, just like notions of truth hood and falsehood or logical implication. The notion of probability relation can’t be broken down further.
The aim of the first chapter:
Its object has been to emphasise the existence of a logical relation between two sets of propositions in cases where it is not possible to argue demonstratively from one to the other