In ECHU IV.15.2, John Locke introduces the notion of 'degrees of
assent'. On the basis of this notion he formulates his claim in IV.
16.1 that the degrees of assent should be regulated by the degrees of
probability. This is a fairly common view in modern philosophy; you
can call to mind, for instance, Hume's famous saying, "The wise man
proportions his belief to the evidence," which Hume means quite
literally: the degree of belief should match the degree of evidence.
http://branemrys.blogspot.com/2004/12/are-there-degrees-of-assent.html
So, apart from the few important things that we can know for certain,
e.g. the existence of ourselves and God, the nature of mathematics and
morality broadly construed, for the most part we must lead our lives
without knowledge. What then is probability? Locke writes:
As Demonstration is the shewing of the agreement or disagreement of
two Ideas, by the intervention of one or more Proofs, which have a
constant, immutable, and visible connexion one with another: so
Probability...
