When I see, for instance, a Billiard-ball moving in a straight line towards another; even suppose motion in the second ball should by accident be suggested to me, as the result of their contact or impulse; may I not conceive, that a hundred different events might as well follow from that cause? May not both these balls remain at absolute rest? May not the first ball return in a straight line, or leap off from the second in any line or direction? All these suppositions are consistent and conceivable. Why then should we give preference to one, which is no more consistent or conceivable than the rest? All our reasonings [sic] a priori will never be able to shew [sic] us any foundation for this preference. - David Hume, 'An Enquiry Concerning Human Understanding 1748
A philosophical skeptic, as evident in the quotation above, Hume (disingenuously I’d wager) didn’t believe in cause and effect. Generally when we are trying to explain a phenomenon, we search for its cause. If we find the cause, we have then explained why the phenomenon has come to be, or why it is the case. This seems simple enough. Science, at the very least, is based on a belief in cause and effect, or so common sense would have us believe. Hume argued that we are actually mistaken in our belief about cause and effect. If we observe that event A is followed by event B, we may feel that there must be some property of A which causes B, but all we actually see is the contiguity and succession of the events.
We have no other notion of cause and effect, but that of certain objects, which have been always conjoin'd [sic] together, and which in all past instances have been found inseparable. We cannot penetrate into the reason of the conjunction. We only observe the thing itself, and always find that from the constant conjunction the objects acquire an union in the imagination. - David Hume, A Treatise of Human Nature 1740.
We cannot actually say that one event caused another. All we know for sure is that one event is correlated to another. For this Hume coined the term 'constant conjunction'. That is, when we see that one event alway 'causes' another, what we are really seeing is that one event has always been 'constantly conjoined' to the other. As a consequence, we have no reason to believe that one caused the other, or that they will continue to be 'constantly conjoined' in the future. The reason we do believe in cause and effect is not because cause and effect are the actual way of nature; we believe because of the psychological habits of human nature.
(On first hearing this, I asked my lecturer what he thought Hume would do if I pointed a loaded gun at Hume’s head. He didn’t answer.)
This is quite a profound philosophical problem. Personally, I can't stand it. It's amazingly unsatisfying. Literally giving up. If someone asks you why something happens, all you can do is shrug your shoulders and say: "I dunno?" I call this the "sh*t happens" account (pardon my descent into scatology). There is no way to explain why something occurred, or to predict when it will occur again. "Sh*t just happens man!"
One of the best solutions to Hume’s radical skepticism comes from Wesley Salmon. The following (admittedly simplistic summary) is from the wikipedia entry on Salmon to which I contributed. (Can one plagiarise oneself?)
A major feature of Salmon’s work consists of providing a philosophically sound basis for scientific explanation. Salmon claimed that a scientific explanation is the state of affairs of something fitting into or being a part of a pattern in the world, where the pattern is constituted by at least one causal process. A process is the real physical connection between cause and effect. For example, the heat from the flame of a gas stove excites the molecules in the water via the iron atoms in the bottom of the pan. This is a process where every step from the cause, the flame, can be traced to the effect, boiling water. This view has come to be known as the ‘ontic’ account of explanation. It is thoroughly realist and rests on an ontology that is designed to answer the question: "Just what is a causal process?"
Salmon argued that events are intersections of two or more causal processes. This interpretation leads to depicting the world as a network with lines (causal processes) and nodes (intersections of causal processes - events). Thus events, causes and effects are reduced to causal processes, where causal processes are real connections between events. According to Salmon, causal processes transmit 'structure', or energy and momentum or 'information' from one spatio-temporal location to another. There are two ways, in principle, by which it is possible to demarcate causal processes from 'pseudo-processes' – how causal processes are transmitted through space-time and what they transmit. As for how causal processes are transmitted, the theory holds that the transmissions must be continuous, with no discontinuities or 'jumps' in space-time. Unfortunately this is not enough to be sure that it is a genuinely causal process, as 'pseudo-processes' can also be continuous.
Demarcation requires that it is necessary to examine what causal processes transmit. Salmon argued causal processes transmit information, and as such we should be able to 'mark' a process by modifying it, to see if the modification is transmitted. This 'marking principal' serves to demarcate causal process from pseudo-processes, as the latter cannot be marked. Further to this, 'things' are 'causal agents' if they are originators of causal chains and not merely 'passers on' of causes. They are 'uncaused causers'. The 'uncaused causer' is the first cause in a new causal chain. It is the initial cause in the chain of causal events we are interested in explaining. The uncaused causer obviously had a cause of its own, but in terms of the phenomenon that we are attempting to explain, the uncaused causer's cause is irrelevant
Salmon's ontology of causal processes seems to fit scientific explanations very well. For example, genes are very much 'causal agents'. Geneticists have inserted the 'antifreeze' genes from flounders into the genetic code of tomatoes. This then protects the tomatoes from frost damage. A process (normal reproduction) has been marked, such that the information that is transmitted (the DNA) is modified (the addition of the flounder gene). Genes, therefore, are a part of a genuine causal process. Specifically, genes are made up of DNA. Inside the cell nucleus, a particular nitrogen-base sequence of DNA controls precisely what proteins are formed in the cytoplasm. By controlling the synthesis of proteins, DNA determines what chemical reactions take place in the cell. The chemical reactions of cells affect the chemical reactions of the body. A small chemical change to the way a particular molecule forms can produce a considerable effect on the phenotype.
Here we have a detailed account of the way genes cause phenotypic effects. DNA is information that tells the cell what kind of proteins to. This, in turn, governs the chemical reactions in the cell, which then produces a phenotypic effect. The gene is the 'uncaused causer' in the chain of causal events we are trying to explain.
Cop that Hume!