Hume argued that
inductive reasoning and belief in
causality cannot be justified rationally; instead, they result from custom and mental habit. We never actually perceive that one event causes another but only experience the "
constant conjunction" of events. This
problem of induction means that to draw any causal inferences from past experience, it is necessary to presuppose that the future will resemble the past, a presupposition which cannot itself be grounded in prior experience.
[13]
An opponent of philosophical
rationalists, Hume held that passions rather than reason govern human behaviour, famously proclaiming that "
Reason is, and ought only to be the slave of the passions."
[12][14] Hume was also a
sentimentalist who held that ethics are based on emotion or sentiment rather than abstract moral principle. He maintained an early commitment to naturalistic explanations of moral phenomena and is usually taken to have first clearly expounded the
is–ought problem, or the idea that a statement of fact alone can never give rise to a
normative conclusion of what
ought to be done.
[15]
Hume also denied that humans have an actual conception of the self, positing that we experience only a
bundle of sensations, and that the self is nothing more than this bundle of causally-connected perceptions. Hume's
compatibilist theory of
free will takes
causal determinism as fully compatible with human freedom.
[16] His views on
philosophy of religion, including his rejection of miracles and the
argument from design for God's existence, were especially controversial for their time.