According to Hume, there are two types of beliefs, relations of ideas and matters of facts. Relations of ideas are indisputable. Such as a widow is a woman whose husband died. Such thoughts are usually definitions. Since it is impossible for a Widow to be anything other then the definition, these ideas are indisputable. Matters of facts claim that if the opposite is imaginable, then it is possible. Matters of fact are debatable, such as the belief in a God or that the world will end.
While it is true that these abstract ideas are easily debatable, other ideas that we held as true are also only matters of fact, such as putting wood in a fire will make I burn. While we hold that it is true that everything falls towards the earth, and that the sun rises, it is possible that the sun will not rise and that things will not fall towards the earth, these beliefs are matters of fact because we can visualize the opposite occurring Hume denies reason any power because he is an empiricist.
Instead three main principles exist that help humans form ideas; they are resemblance (when looking at a picture a person thinks of the object), contiguity (thinking of an object that is close spatially), and cause and effect (association). Hume claims that reason alone cannot establish matters of facts. There is no reason to believe that what happened one time will happen again. For example, there is no reason for Adam to believe that a rock will fall if he drops it unless he experiences it many times. Even with experience one cannot reason a matter of fact to be true, because the universe may not be uniform.
There is a chance that because one thing happened many times, it makes it more possible that it will not happen again. Hume gives a very possible argument for why the universe may not be uniform. He claims that all beliefs are either arguments based on relation of ideas (such as definitions) or arguments based on experience (such as matters of fact). All arguments based on experience require a uniformity of nature principle. In order to argue that putting wood in a fire makes it burn, someone must do the same action many times, but even then there is no reason to believe that the wood will not burn, but extinguish the fire instead.
There is a chance that wood actually extinguishes fires, but once in a while it will just burn instead. Unless nature is uniformed then there would be no reason for anyone to believe that wood will burn. The uniformity of nature cannot be proved or based on experience. If based on experience, a circular argument is formed. Therefore there are no reasons for believing that nature is uniform. Therefore no arguments based on experience are reasonable.