David Hume
Principles of association

Regardless of how boundless it may seem; a person's imagination is confined to the mind's ability to recombine the information it has already acquired from the body's sensory experience (the ideas that have been derived from impressions). In addition, "as our imagination takes our most basic ideas and leads us to form new ones, it is directed by three principles of association, namely, resemblance, contiguity, and cause and effect":

Hume elaborates more on the last principle, explaining that, when somebody observes that one object or event consistently produces the same object or event, that results in "an expectation that a particular event (a 'cause') will be followed by another event (an 'effect') previously and constantly associated with it". Hume calls this principle custom, or habit, saying that "custom...renders our experience useful to us, and makes us expect, for the future, a similar train of events with those which have appeared in the past". However, even though custom can serve as a guide in life, it still only represents an expectation. In other words:

Experience cannot establish a necessary connection between cause and effect, because we can imagine without contradiction a case where the cause does not produce its usual effect…the reason why we mistakenly infer that there is something in the cause that necessarily produces its effect is because our past experiences have habituated us to think in this way.

Continuing this idea, Hume argues that "only in the pure realm of ideas, logic, and mathematics, not contingent on the direct sense awareness of reality, causation safely…be applied—all other sciences are reduced to probability". He uses this scepticism to reject metaphysics and many theological views on the basis that they are not grounded in fact and observations, and are therefore beyond the reach of human understanding.
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