Rationalists think synthetic claims can be justified by employing a priorireasoning. For example, Descartes thinks God is no deceiver. What is the...

Rationalism
is a school of thought in Western philosophy which distinguishes between empirical knowledge and
knowledge attained through reason. Knowledge that is attained solely through reason is
a priori knowledge, as it is prior to ones own experiences. A
posteriori knowledge, meanwhile, is knowledge derived from ones own
experiences.

Empiricism is a school of thought in Western philosophy which
rejects the distinction between a priori and a
posteriori knowledge and maintains that only a posteriori
knowledge truly exists. To assert the dominance of a posteriori knowledge,
empiricists established the analytic-synthetic distinction: analytic propositions are
propositions whose truth can be verified simply by knowing the meaning of its constituent terms,
while synthetic propositions cannot be verified unless one knows how its meaning relates to the
material world. Through this model, so-called a priori statements are
proven to be merely analytic propositions. As their meaning relies on their constituents, they
are therefore tautological and so offer no new knowledge.

Additionally,
analytic a priori propositions are held to be necessary, as they are either
necessarily true or necessarily false. Meanwhile, synthetic a posteriori
propositions are held to be contingent, as their truth depends on more informationthey are
neither true under every possible valuation nor false under every possible
valuation.