Summary: A Treatise of Human Nature (page 2)

Part 1: Of Ideas, their Origin, Composition, Connection, Abstraction, etc.

The first part of Book I is devoted to ideas: What are the ideas in our minds? What is their origin? How are they formed?

To answer these questions, Hume begins by distinguishing ideas from impressions—an essential distinction made in the very first line of the chapter:

All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call IMPRESSIONS and IDEAS ¹

Impressions and ideas are differentiated by their degrees of force and liveliness, with which they strike upon the mind ².

Hence this definition:

Those perceptions, which enter with most force and violence, we may name impressions; and under this name I comprehend all our sensations, passions and emotions, as they make their first appearance in the soul.
By ideas, I mean the faint images of these in thinking and reasoning; such as, for instance, are all the perceptions excited by the present discourse ³.

For Hume, this distinction and these definitions pose no difficulty: they correspond to the fundamental and self-evident difference between two faculties of the mind—feeling and thinking.

I feel hot: this perception is an impression, imposing itself with all its force upon my mind.

I imagine that I am hot: this perception is an idea, a weakened image of the original sensation, which means I do not actually suffer from the heat.

Hume introduces a second major distinction, this time concerning the notions of simplicity and complexity. Simple impressions are such as admit of no distinction nor separation, ⁴ whereas complex ones may be distinguish'd into parts ⁵. The same applies to ideas.

Thus, the impression (or idea) of an apple is complex: its flavour is distinct from its colour, texture, and other properties.

These two conceptual pairs—impression/idea and simple/complex—will be sufficient for Hume to construct an innovative theory of knowledge.

It seems that the mind is somehow struck by redundancy: All the perceptions of the mind are double, and appear both as impressions and ideas ⁶.

Upon closer examination, however, we find that some complex ideas do not correspond to any impression—such is the case, for example, of an imaginary city.

Conversely, some complex impressions in my mind do not correspond to any idea. Take the city of Paris: I have seen Paris; but shall I affirm I can form such an idea of that city, as will perfectly represent all its street and houses in their real and just proportions? ⁷

Impressions and complex ideas resemble each other, yet they are not exact copies. On the other hand, any simple idea is merely a copy of a simple impression (for example, the colour red).

Let us now turn to a genetic explanation: How are our ideas formed?

Since every simple impression is attended with a correspondent idea ⁸ and vice versa, we are faced with an alternative: it proves a dependence of the impressions on the ideas, or of the ideas on the impressions. How can we know on which side this dependence lies? ⁹

Are we to believe that the idea precedes the corresponding impression, or is it the other way round?

To answer this question, we simply have to rely on experience. What does it reveal?

I consider the order of their first appearance; and find by constant experience, that the simple impressions always take the precedence of their correspondent ideas, but never appear in the contrary order. To give a child an idea of scarlet or orange, of sweet or bitter, I present the objects, or in other words, convey to him these impressions; but proceed not so absurdly, as to endeavour to produce the impressions by exciting the ideas. ¹⁰

Likewise, if the impression does not exist, the idea does not appear: thus, a blind person has no idea of light.

Here, then, is the foundation of this general proposition: All our simple ideas in their first appearance are deriv'd from simple impressions, which are correspondent to them, and which they exactly represent ¹¹.

This principle of the priority of impressions to ideas ¹² aligns with the empiricist tradition: it strikes down the claims of reason, which, far from reigning supreme as in the rationalism of Leibniz, appears only as derived and dependent.

Section II introduces a new distinction: between impressions of sensation and impressions of reflection.

The former correspond to sensations, while the latter include passions, desires, and emotions.

Here is the general mechanism for the formation of impressions of reflection:

An impression first strikes upon the senses—heat, hunger...

Of this impression there is a copy taken by the mind, which remains after the impression ceases; and this we call an idea. For example, the idea: I am hungry.

This idea, in turn, produces the new impressions of desire and aversion, hope and fear—which are precisely the impressions of reflection.

These again are copy'd by the memory and imagination, and become ideas; which perhaps in their turn give rise to other impressions and ideas ¹³. For example, I am afraid I will be hungry soon.

Hume dismisses the study of sensations, as they fall within the domain of other sciences—anatomy and natural philosophy. His concern, it should be remembered, is moral philosophy.

We must therefore focus on reflective impressions. Since these arise mostly from ideas ¹⁴, we must begin with ideas.

How does an impression become an idea? Through memory and imagination.

Memory, when the impression retains a considerable degree of its first vivacity. Imagination, when the impression entirely loses that vivacity ¹⁵.

While memory has the advantage of retaining the original order of impressions, imagination is not restrain'd to the same order and form with the original impressions, and is therefore free to transpose and change its ideas ¹⁶.

¹ 1.1.1, p.7
² Ibid.
³ Ibid.
⁴ Ibid.
⁵ Ibid.
⁶ p.8.
⁷ Ibid.
⁸ P.9
⁹ Ibid.
¹⁰ Ibid.
¹¹ Ibid.
¹² P.10
¹³ 1.1.2, p.11
¹⁴ Ibid.
¹⁵ 1.1.3, ibid.
¹⁶ P.12