....Causes and effects are discovered, not by reason but through
experience, when we find that particular objects are constantly
conjoined with one another. We tend to overlook this because most
ordinary causal judgments are so familiar; we've made them so many
times that our judgment seems immediate. But when we consider the
matter, we realize that "an (absolutely) unexperienced reasoner could
be no reasoner at all" (EHU, 45n). Even in applied mathematics, where
we use abstract reasoning and geometrical methods to apply principles
we regard as laws to particular cases in order to derive further
principles as consequences of these laws, the discovery of the
original law itself was due to experience and observation, not to a
priori reasoning.
Even after we have experience of causal connections, our conclusions
from those experiences aren't based on any reasoning or on any other
process of the understanding. They are based on our past experiences
of similar cases, without which we could draw no conclusions at all.
But this leaves us without any link between the past and the future.
How can we justify extending our conclusions from past observation and
experience to the future? The connection between a proposition that
summarizes past experience and one that predicts what will occur at
some future time is surely not an intuitive connection; it needs to be
established by reasoning or argument. The reasoning involved must
either be demonstrative, concerning relations of ideas, or probable,
concerning matters of fact and existence...
....all of us - ordinary people, infants, even animals - "improve by
experience," forming causal expectations and refining them in the
light of experience...
....When we examine experience to see how expectations are actually
produced, we discover that they arise after we have experienced "the
constant conjunction of two objects;" only then do we "expect the one
from the appearance of the other." But when "repetition of any
particular act or operation produces a propensity to renew the same
act or operation...we always say, that this propensity is the effect of
Custom".
So the process that produces our causal expectations is itself causal.
Custom or habit "determines the mind...to suppose the future conformable
to the past." But if this background of experienced constant
conjunctions was all that was involved, then our "reasonings" would be
merely hypothetical. Expecting that fire will warm, however, isn't
just conceiving of its warming, it is believing that it will warm.
Belief requires that there also be some fact present to the senses or
memory, which gives "strength and solidity to the related idea." In
these circumstances, belief is as unavoidable as is the feeling of a
passion; it is "a species of natural instinct," "the necessary result
of placing the mind" in this situation.
http://plato.stanford.edu/entries/hume/
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