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H1. Hume's Theory of Ideas

Hume like Locke before him believes the mind starts as a blank slate. All our ideas come to us from impressions made by the senses and the reflections of our mind. His theory of ideas is atomistic and his empirical method built on two guiding principles: firstly, that all ideas are copies of impressions and secondly, that whatever distinguishable ideas we have are separable and vice versa. This leads him away from a rationalist metaphysics that relies upon a faculty of intellect. Instead, he grounds his theory on observation leading to a reductionist and modest approach of defining our conceptual understanding.

Perceptions, the Contents of the Mind

Hume defines mental content as perceptions which are either impressions or ideas. The distinction is central to his theory of ideas and his method of cognitive psychology. 

Impressions are distinguished from ideas by their force and vivacity, the difference between feeling something and merely thinking it (THN 49). There are two sources of impressions: sensations whose causes are ultimately unknowable to us and reflection which is derived from the operations of the mind on ideas. 

Ideas are images, weak copies of impressions and there are no innate ideas (THN 54). Where we suspect some idea in our mind of having no meaning, Hume asks us to check from which impression it is derived. And if there is no corresponding impression, to discard it as meaningless (E 22).

Simple and Complex Ideas

Hume further classifies ideas into simple ideas which cannot be separated and complex ideas which are composed of simpler parts (THN 50). All simple ideas are exact representations of simple impressions. Complex impressions, however, are never copied directly to complex ideas (THN 51).

For example, the ideas of heat and cold are copied from their corresponding sensations. Whilst pleasure and pain are ideas derived from reflection (THN 55).

Memory and Imagination

There are two mental faculties that handle ideas: Memory preserves the order and position of simple ideas, whilst imagination has liberty to transpose and change ideas at will (THN 57, G 26-27).

Vivacity and force are the only qualities that distinguish impressions, memories and ideas (THN 56, G 13). Hume allows some overlap between the three. Some memories are as faded as ideas, some ideas are so vivid they could be mistaken for impressions (G 26-27).

The imagination plays an essential role in our thinking but is constrained to work with ideas derived from experienced impressions (E 19). It is the faculty by which ideas are combined and separated (G 13).

There are two senses in which the imagination is used in Hume's texts (G 27). In the first, imagination is the faculty by which we are moved by argument to belief (aka inference). In the other sense, imagination is vivacious in its ideas and requires no intermediate steps, no reasoning, to be convincing. This latter sense is what Hume associates with witnessing miracles and being in proximity to religious relics (G 28-29).

Hume, like Berkely and Locke, dispenses with the notion of a faculty of intellect, where non-imagistic thoughts, such as God or infinity, are represented (G 14-15). There are no necessary truths available to the intellect nor does Hume unlike Leibniz believe ideas from the senses are by nature confused (G 17). Hume believed all our ideas are copied from impressions in a clear and precise manner, having the same nature as our impressions. There are no spiritual perceptions that could only be interpreted by pure intellect. According to Hume, such reasoning is spurious fiction designed to cover up absurdities in the metaphysics of some philosophers (G 22-23).

Relationship of Ideas

Ideas have three types of relationships under Hume's theory. All three types of relationship are transitive, i.e. if A is related to B and B is related to C, A is also related to C (THN 59).

Resemblance is necessary for the comparison of objects (THN 61), e.g. a dog resembles a cat in that they are both mammals but differ in that dogs bark whilst cats meow.

Identity is only strictly true for constant, unchanging objects. However, we take any object that has existed for any duration as being identical with itself in the past (THN 58).

The causal relation is the most common of the three relations (THN 59). Hume distinguishes between actual causation, e.g. the white ball hits the red ball, and potential causation, e.g. the match has the potential to start a fire. Importantly, Hume is committed to a causal explanation for the natural world including human cognition.

Quantity and Quality

Hume argues that all impressions come with both quality and quantity determinants. Therefore, all ideas also come with quantity and quality (THN 65-66). Both quality and quantity are always accompanied by a numeric value or degree (THN 67). 

We simply cannot think of an object without thinking of it in some quantity or with one or more qualities.

Contrariety, Substance and Mode

There are no contrary ideas except that of existence and non-existence, which is obtained via the experience of causation (THN 62). Something may cease to be or come to be but cannot be both existing and not existing at the same time. 

Hume rejects the Aristotlean theory of substance, the essence of an object without its properties. For him, the concept is derived from reflection rather than sensation and is merely a collection of qualities given a single name (THN 63). He, therefore, rejects explanations of change and sameness based on substance, preferring empirical comparison by degrees of quality and quantity to determine equality.

For the same reasons, modes of ideas, defined by Descartes as attributes which modified substances, are to Hume collections of simple ideas united by the imagination and given a name (THN 63). 

As Hume says, "All general ideas are nothing but particular ones, annexed to a certain term" (THN 64). There simply is no access to an underlying reality of substances and the pretence that there is has led to absurdities.

Abstract Ideas

"Some ideas are particular in their nature, but general in their representation" (THN 69).

Where there is a resemblance between several objects, we give them the same name regardless of the degrees of difference in their quantity or quality (THN 67). Abstract ideas are thus a custom of the mind, derived from experience, that raises individual ideas and annexes to them several words or reasonings, making them a general idea (THN 68-69). 

A particular abstract idea is used in reasoning for other resembled ideas, irrespective of the differences in circumstances (THN 71). For example, figure and colour are used as distinctions of reason, even if we never obtain a simple impression that can separate figure or colour. Instead, we always see them as qualities of some object (THN 72).

Hume rejects Locke's indeterminacy about the representation of abstract ideas, instead, he uses the Copy Principle (see below) to ground the process of abstraction in the imagination (G 23).

Space, Time and Existence

Three key examples of abstract ideas are the notions of space, time and existence (THN 75-77, G 52-53). 

Space is a complex idea composed of the visual and tactile simple impressions we experience. 

Hume rejects the Lockean notion of time being a succession of moments. Instead, time is a complex idea that revives our disposition to recall other temporally complex ideas. 

Likewise, Hume rejects Locke's separate and distinct idea of existence. Instead, existence is an idea we get from every impression of an object.

Empty Space and Changeless Time

Hume denies there is any impression of vacuum to serve as an abstract idea. Instead, he solves the problem of the possibility of motion by having spatially non-dense, non-contiguous bodies without alluding to a non-perceivable vacuum (G 54).

In the same way, he denies we can have any real conception of changeless time as we see time as a succession of changing bodies (G 55).

Problems with Hume's theory of abstract ideas

Hume is oversimplifying the nature of mental concepts by restricting human cognition to a purely imagistic identification of impressions that lead to ideas. It is doubtful his method of focusing on the images of our experience is the most effective approach to improving our conceptual understanding (G 57).

More troubling perhaps is that Hume's theory doesn't account for errors. There are no wrong generalisations we can make or at least no way to know how counter-examples affect our generalisations under his theory (S 39-41).

The Copy Principle

The key principle that all simple ideas are copied from impressions is used time and time again as the basis of Hume's reasoning (THN 4, E 13). 

This principle can be divided into two theses (G 4):
  1. Every simple idea is derived from a simple impression (the Causal Thesis or Temporal Hypothesis in S 24).
  2. All simple ideas resemble exactly their simple impression (the Resemblance Thesis).
Therefore, to perfectly understand an idea we have to trace it back to its first impressions, otherwise, the idea doesn't exist in reality (E 22, G 41).

One counter-example Hume offers against his own theory is about a missing shade of blue in a spectrum of blues. Our imagination seems to derive the idea of the shade without there being a corresponding first impression (THN 5-6, E 16). Hume explains this by allowing natural resemblance of simple perceptions to fill in the missing shade.

Hume's first principle isn't derived a priori nor is it logically necessary. It an empirical thesis of experience based on the constant conjunction of impressions and ideas which Hume supports with evidence and examples (S 27, G 45-47). 

Likewise, his methodological use of the principle isn't sufficient to argue for the existence or non-existence of certain concepts but only to show that in some cases no such impressions can be found, e.g. vacuum or unchanging time (G 48-50). 

An important outcome of the first principle is that we cannot control the ideas that form in our minds. They come to us from our impressions unbidden and against our wills (S 30). 

The Separability Principle

The second important principle Hume deploys is that whatever objects are distinguishable are separable and likewise, whatever is separable is distinguishable (THN 18, G 23-24). Simple impressions and their corresponding ideas admit of no separation and thus make up the atomic elements of Hume's theory (G 66-67).

Conceivable Criterion of Possibility

Hume argues that nothing we can conceive clearly and distinctly is absurd and impossible (G 24). This is not to argue that whatever we can conceive exists, only that it is possible for it to exist.

Whatever the imagination can conceive as being different, can be separated and transposed and changed in the mind (G 68-69)

Necessary Connections

Hume argues that based on the separability principle we cannot perceive any necessary connection between cause and effect as we can distinguish clearly between both the causing event and its effect. We can, in fact, imagine it being otherwise, i.e. the effect not following the cause (G 64-65). 

This is the opposite of the Cartesian notion of a "real distinction" such that the existence of X may entail the existence of Y (G 69).

Representational Simplicity and Separability

Hume's simple inseparable impressions are atomic 'minimum sensations' and not compatible with the Lockean notion of simple ideas. It takes at least two simple impressions to form a shape, to define an extension in space, and to frame the sensation of duration (G 60-62).

Providing the example of a white marble globe, Hume distinguishes between the form of the body, its motion, figure and colour by use of the separability principle. The globe is a complex impression of distinguishable simple parts. The white colour of each of these simple impressions, arranged in the figure of a sphere. Each simple impression can be a perception of more than one thing, e.g. whiteness, sphericality, the motion of the globe etc. (G 63-64).

Therefore, in Hume's theory, all non-atomic visual and tangible perceptions are complex. His use of abstract ideas avoids constraining his theory such that every feature of perception is a different simple object (G 69-70). This does leave Hume with a problem in accounting for classical geometry which he demotes from being demonstrative knowledge to being merely probable (G 74-75).


G - Garret, Don. 1997. Cognition and Commitment in Hume's Philosophy. New York: Oxford University Press. 3 - 75.

E - Hume, David. 2008 [1748]. Sections 1-3 in An Enquiry Concerning Human Understanding. New York: Oxford University Press.

THN - Hume, David. 1985 [1740]. Book 1, Part 1, Sections 1-7 in A Treatise of Human Nature. London: Penguin. 

S - Stroud, Barry. 1977. "Theory of Ideas" in Hume. New York: Routledge. 17-41.


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