En route to ridding philosophy of 'metaphysical nonsense', Hume reduced Locke's, empiricism to a point of skepticism and went one step further than Berkeley's denial of materialism, by denying the existence of everything - except our perceptions. He once famously said, "I am nothing but a bundle of perceptions".
According to Hume, we really know nothing for sure. Whatever we think we know is based upon our experience of the external world (empiricism), but there is not even rational proof that such a world exists. Furthermore, even if there is an external world, all conceptions of 'cause' and 'effect' relationships, as in claimed scientific study, are impossible to determine. The supposition that an event is followed by a succeeding event, is merely human expectation projected onto reality.
Hume's skepticism was furthered in his assertion that human belief in causation is merely inductive reasoning - the process that leads us to make generalizations from observing a number of similar situations. He claimed that inductive reasoning is not reliable enough to lead us to any particular truth and that all scientific laws are merely generalizations from inductive reasoning.
Along with the position that we can't know that an external world exists, we cannot justify the claim to have knowledge of the existence of God, the human soul or absolute moral values.
In his first and probably best work, Treatise of Human Nature, Hume examines the ways in which we, as human beings, perceive the world. He maintains we have two types of perceptions - impressions, which are all our 'sensations, passions and emotions,' and ideas, which are the 'faint images of these impressions in thinking and reasoning'. These impressions and ideas can be simple or complex. Impressions cause ideas but ideas do not cause impressions. We have the faculty of memory, which retains ideas in order of which they occur, and the faculty of imagination, which can rearrange or combine ideas derived from impressions.
Interestingly, Hume observes that we never experience our own self. He puts forth that we have no conception of the self as a non-material substance existing alongside our flow of thoughts and experiences. Rather, the self is the flow of experiences. His theory of self is often referred to as the 'bundle theory'. In other words, the self is not the non-material substance in addition to the bundle of thoughts and experiences, it is that bundle.
While Locke's empiricism acknowledged an external world separate from our perceptions of it, and Berkeley's denied the existence of material substance, conceding only the existence of minds and their ideas, Hume went further, arguing that there is little reason to assert the existence of either.
Hume's empiricism was one of experience alone. All that we are entitled to say, according to Hume, is that there are briefly lived experiences with neither cause nor object in an outer world and without necessarily any connection between them.
Hume's skepticism and brand of empiricism let go of all previous philosophical claims and presuppositions. It raised important points and questions that hitherto had not been addressed. His work paved the way for progress and the search for truth in a new and different direction.