According to David Hume, causes are invariably followed by their effects: “We may define a cause to be an object, followed by another, and where all the objects similar to the first, are followed by objects similar to the second.” (1748, section VII.) Attempts to analyze causation in terms of invariable patterns of succession are referred to as “regularity theories” of causation. There are a number of well-known difficulties with regularity theories, and these may be used to motivate probabilistic approaches to causation.
http://plato.stanford.edu/entries/causa ... abilistic/