The question of the origin of human beliefs is included in what Hume calls ‘the science of man’. This science, the theory of human nature, is a foundation for all other sciences, both natural and moral. (By moral science or moral philosophy Hume does not mean just the study of morality, but also of politics, aesthetics, history.)

There is no question of importance, whose decision is not compriz’d in the science of man; and there is none, which can be decided with any certainty, before we become acquainted with that science. In pretending9 therefore to explain the principles of human nature, we in effect propose a compleat system of the sciences, built on a foundation almost entirely new, and the only one upon which they can stand with any security.10

Hume’s theoretical explanation of our beliefs about the causal properties of objects begins with his account of experience – it is from experience and not from a priori reflection that we discover that fire burns us. That is hardly news; but what gives Hume’s account part of its distinctive character is his conception of experience. This conception has its origin in the thought of some of his predecessors, especially Descartes, Nicolas Malebranche (1638–1715), John Locke (1632–1704) and George Berkeley (1685–1753).11 Experience is characterized subjectively, from the point of view of the person having the experience, and is a matter of the immediate content of consciousness. Our experience of fire, say, consists essentially in our having before our minds a perception of fire. Hume makes clear that his talk of perceptions in the mind as constituents of experience is part of a scientific theory. He says that ‘the vulgar confound perceptions and objects’. Unreflective, ordinary men and women fail to distinguish between the fire and the perception of fire. They think that, in their experience of fire, the fire itself is a constituent of their consciousness and is, in some way, immediately before the mind. But here, as elsewhere, common assumptions and science diverge: ‘… philosophy informs us, that everything, which appears to the mind, is nothing but a perception.’12

Consciousness, for Hume, has two forms, feeling and thinking. When we feel, the warmth of the fire, the sensation of warmth, the sight of the flames, and the pleasure we get are all perceptions of the kind he calls ‘impressions’. When we think, as now, of such a situation, the perceptions before our minds are called ‘ideas’. Ideas, he argues, are copies of and originate from impressions. This aspect of Hume’s theory has many ramifications. But one consequence is the denial that we can have ideas which, so to speak, go beyond the basis of impressions. This thesis is employed in the Dialogues by Philo to support the view that the nature of God is incomprehensible. Strictly speaking, we can have no idea of the divine nature:

Our ideas reach no farther than our experience. We have no experience of divine attributes and operations. I need not conclude my syllogism: You can draw the inference yourself.13

Although the content of our ideas cannot outrun the materials provided by experience, there is an obvious and important way in which we constantly think of things we have not experienced. If I see a child put her hand in the flame (I have that impression), I shall at once think that she will be burned (I have that idea). This idea, that she will be burned, is not derived from seeing her being burned. I think of what will happen before I see it happen. Anyone who has had the same kind of experiences of fire as I have had will do the same. This mental process (Hume calls it an ‘operation of the mind’) is a universal feature of human nature. It is essential to our life. It is a major task of the theory of human nature to explain this process. Hume regarded himself as the first person ever to do so successfully.

We must note at once that the question is not, why does fire burn? It is obvious that we all learn to make such inferences, from what we experience to what will happen next, long before we have any scientific understanding of the processes of nature. For Hume, what we do is to infer a cause or an effect (in the example, the effect of being burned) from an effect or a cause that we currently experience.