I am looking for the source of the following quotation:

In the web, all our beliefs are justified by all our other beliefs,
they are connected by an explanatory network, and changes in one place
can require changes elsewhere. Thus all belief is connected to
observation in the world. Are any beliefs immune from this process?
Some beliefs do not depend on observation for their justification, in
fact no observation whatever could show them to be wrong. Beliefs of
this type are said to count as a-priori knowledge: Their justification
is independent of experience, a-priori knowledge is contrasted with
empirical knowledge which does depend on observation for its
justification.

This is for a friend of mine and I do not know the context for the request.