Impressions and Ideas

The purpose of this paper is to attempt to illustrate, in a non-technical and a rather superficial way, one of the most important notional divisions in the philosophy of David Hume: Impressions and Ideas. To be more precise, I provide an introductory explanation of the main difference between two conflicting interpretations of Hume’s criterion distinction, one of which, I believe, is incomplete, if not wrong, and the other also incomplete but potentially correct. I take that Jonathan Bennett among others holds the first interpretation, while David Landy the second one. It recognizes that the incompleteness of both readings is undoubtedly encouraged by Hume’s own ambiguities that give rise to consternation and dispute among his readers, dictated primarily by his hastiness and even more by that of his interpreters.

In relation to his 17th century predecessors, Hume inaugurates a newly critical relationship to Philosophy, more correctly to Metaphysics, its illusions and its Cartesian glamorous pretensions. One would not go far astray by saying that Hume brought philosophy to its ‘common’ senses.

Hume as it obvious from his writings is an empiricist and a naturalist, though quite a peculiar one. He thought that all contents of the mind are derived from experience. His epistemology is rooted in the tradition of British empiricism, which is traced to Locke and Berkeley. He generally accepted Locke’s contention that at birth the human mind is a “tabula rasa” or blank slate waiting to be written on by experience. There are no innate ideas within the mind but that its content is derived from experience. Empiricists are opposed to the Cartesian thesis that reason unaided by experience is the truest possible way to know the world. The empiricists take the opposite position, arguing that sensation is the only way one can truly know anything.

As it is obvious from his major philosophical work A Treatise Of Human Nature, Hume wants and thought of himself as developing a science of human nature and understanding on the basis of empirical investigation. He asked the same questions as the previous empiricist had asked. How is knowledge possible? What are the limits of knowledge? According to the empirical tradition of epistemology, the source of knowledge is sensation or, what in Hume’s language became impressions. But let us not put the cart before the horse, and let us take things in a temporal succession.

Don Garret has pointed out that there are four principal distinctions in Hume’s philosophy:

1) Mental entities which Hume calls “perceptions” are divided into “impressions and ideas”

“All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call IMPRESSIONS and IDEAS. The difference betwixt these consists in the degrees of force and liveliness, with which they strike upon the mind, and make their way into our thought and consciousness”.

2) “Simple” and “complex” impressions

“There is another division of our perceptions, which it will be convenient to observe, and which extend itself both to our impressions and ideas. This division is into SIMPLE and COMPLEX. Simple perceptions or impressions and ideas are such as admit of no distinction nor separation. The complex are the contrary to these and may be distinguished into parts”.

3) “Impressions of sensations” and “impressions of reflexion”

“Impressions may be divided into two kinds, those of SENSATION and those of REFLEXION. The first kind arises in the soul originally from unknown causes. The second is derived in a great measure from our ideas, and that in the following order. An impression first strikes upon the senses, and makes us perceive heat or cold, thirst or hunger, pleasure or pain of some kind or other. Of this impression there is a copy taken by the mind, which remain after impression ceases; and this we call an idea. This idea of pleasure of pain, when it returns upon the soul, produces the new impressions of desire and aversion, hope and fear, which may properly be called impressions of reflexion, because derived from it”.

4) The distinction between ‘two idea-forming faculties’ “Memory” and “Imagination”

“We find by experience, that when any impression has been present with the mind, it again makes its appearance there as an idea; and this it may do after two different ways: either when in its new appearance it retains a considerable degree of its first vivacity, and is somewhat intermediate betwixt an impression and an idea; or when it entirely loses that vivacity, and is a perfect idea. The faculty by which we repeat our impressions in the first manner, is called the MEMORY, and the other the IMAGINATION”.

I shall not say anything substantial of the two last divisions, since my main concern is to elucidate the distinction and the connections between impressions and ideas, their division into simple and complex and the Copy principle to the extent that it sheds light on understanding this distinction. I find Hume’s presentation as it is quite ambiguous and there certainly emerge a plethora of questions which cannot be dealt with here in any satisfactory manner.

I shall maintain that despite many sidedness of Hume’s philosophy the building blocks of his philosophy are contained in the four divisions we presented above. By saying this I do not imply that other parts of Hume’s philosophy are less relevant than this division. On the contrary, had Hume been content with this part of his philosophy only, had he relied on our capabilities to built upon those foundations and written nothing more, in all probability we would not have gained any special insight into Humean philosophy. We could simply not infer by reading Part 1 of the Book 1 of Hume’s Treatise all his subsequent philosophy. Since, however, Hume has further developed his philosophy with which we are familiar we can safely say that unless one has a sufficient grasp of the building blocks one cannot hope to accomplish much from reading, say, Hume’s philosophy of causality prior to understanding the four-mentioned division.

On Impressions and Ideas

In Hume’s philosophy the distinction “Impressions” and “Ideas” is the main distinction, the one on which the entire Humean philosophy depends. Our experience begins with what Hume calls impressions. They are subsequently duplicated in the mind, that is, they are copied into ideas. We shall address the Copy Principle below.

I shall only say something of impressions and ideas. This division seems to me to be the foundation of his philosophy, and it explains the reasons behind Hume’s decision to take such a controversial position as he does in matters concerning, to mention just one example, the idea of the necessary connection between cause and effect. Once we get an idea of how the Copy Principle works, we have basically got into the heart of Hume’s philosophy. Let us see what Hume wants to say, or what his successors have made him say.

There are two main interpretations of this distinction (Impressions and Ideas) in Hume’s philosophy. The first interpretation, which, as far as I could gather from my readings, most philosophers accept, maintains that according to Hume, the total content of the mind consists of perceptions. Perceptions are of two kinds: “impressions” and “ideas”. Impressions are those perceptions which are more lively and forceful, and they include sensations and emotions. The faint images of these impressions Hume terms ideas. Impressions (sensations and emotions) are experienced; ideas (faint images of sensations and emotions) are revived in imagination and memory.

“Perceptions” thus form the total contents of our mental states, and they are all we can know. Impressions and Ideas are species of the genus perception. “All the perception of the human mind resolve themselves into two distinct kinds, which I shall call impressions and ideas” declares Hume in his Treatise.

But what are impression and ideas? By what criterion do we distinguish between them? First Hume assumes that distinction does not need much explanation. We know there is a difference between actually perceiving something and just thinking about that thing in its absence. And this difference is the difference between having an impression of something and having an idea of something. The distinction between impressions and ideas is simply a distinction between the degrees of force and liveliness with which perceptions strike upon the mind. There is plainly a difference between the sensation of experience of heat or cold, which is an impression, and thinking about the sensation of heat later in time when we are no longer experiencing it, which is an idea. Or to take another obvious example, the difference between being/feeling angry and thinking that same emotion. To put it simply, though somewhat misleadingly, it is the difference between feeling and thinking.

This is the basic furniture of our mind. All perceptions of the mind are double, and appear both as impressions and ideas. That is, when we inspect our minds we find that all contents come in pairs, and the only difference between the members of each pairs is a difference in the degree of force and vividness with which they strike upon the mind.

There are exceptions to this doctrine. It is not difficult to imagine counterexamples, cases in which an idea may turn out to be livelier than the impression from which it is copied. The important point for Hume, however, is not these exceptions but the fact that impressions, as a norm, are more vivid and always precede the ideas which are fainter, that impressions are the originals of which ideas are the copies.

Impressions and ideas, as we mentioned previously, can be simple and complex. Simple ideas and impression admit of no distinction or separation, they cannot be broken down or be analysed any further. Complex ideas on the other hand may be divided into parts. A complex impression is composed of a group of simple impressions. An apple, for example, is a complex impression. It has a particular colour, odor, a certain taste, etc. Or to take another example, the concept RED PENCIL is complex and contains the concept RED and PENCIL. Likewise, complex impressions have simple impressions as their parts.

All Simple ideas are copies of simple impressions and that complex ideas reduce without residue to the simple ones that are their constituents. All complex perceptions are made of simple perceptions. The claim is exhaustive, implying that there is nothing at all in the cognitive mind except sensations and what is derived from them. Even the idea of God arises from reflecting on the operations of our own mind. He calls the relation a “Universal resemblance” and “constant conjunction”. Hume is so convinced of the copy principle that he even issues a challenge to those who doubt the principle:

“If one should deny this universal resemblance, I know no way of convincing him, but by desiring him to show a simple impression, that has not a correspondent idea, or a simple idea, that has no correspondent impression. If he does not answer this challenge, as it is certain he cannot, we may from his silence and our own observation establish our conclusion”

(p. 3,4).

For every simple idea there is a corresponding simple impression. This conjunction holds so universally that it cannot be due simply to chance. There must be some connection between things of the two ‘kinds’. Whenever we find a correlation like this we can determine the direction of the causal link by finding out which of the two kinds of things always occur first in time.

We must, however, be more specific with regard to the distinction in question. Not how ideas differ from impressions but how an idea differs from its corresponding impressions. What is it that belongs to one class rather than another? We have already seen the answer that Hume gave. They differ only in the degree of force and liveliness with which they strike upon the mind. If we find that the member of the pair, which strikes with greater force, is present, then we are having an impression, and if we find the opposite then we are having an idea. It is the same as the distinction between perceiving and feeling something and thinking about it.

Is the difference between perceiving and thinking simply a difference between the degrees of force and liveliness with which certain objects strike upon the mind? Obviously, everyone will acknowledge that there is a difference between perceiving/feeling something and thinking about it. Hume, however, is putting a view about what the difference is. Hume, suggests Barry Stroud, is not helpful in this regard. “The obviousness of the fact that there is a distinction between perceiving and thinking does not make Hume’s account of that difference obvious. In fact, it is not even clear, what his account comes to. If it is taken fairly literally it does not seem to be very plausible”.

The obscurity of Hume’s account of the distinction is obvious in the very definition that he gives to this distinction. Let us quote Hume again and examine more closely where the confusion lies. Hume says:

“All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call IMPRESSIONS and IDEAS. The difference betwixt these consists in the degrees of force and liveliness, with which they strike upon the mind, and make their way into our thought and consciousness”.

A careful reading of this passage reveals that there is an ambiguity, dichotomy even, in Hume’s understanding of this distinction. I have underlined the words which cause the confusion and ambiguity. Does Hume maintain that impressions and ideas differ only in kind or only in degree or both? The passage above maintains that impressions and ideas differ in kind, and also in degrees of force and liveliness. The bulk of Hume’s ensuing comments, however, seem to substantiate the claim that the difference is merely one of degree.

Norman Kemp Smith has noticed the dichotomy and maintains, rightly in my opinion, that the two accounts are inconsistent. “What renders Hume’s statement obscure and bewildering,” says Smith, “is the twofold manner in which the difference is formulated, as being at once difference of kind and yet also difference that admits of degree. The two ways of regarding it, so little compatible with one another, are almost equally emphasized” .

David Landy has also problematized and contested the first interpretation. He says that although Hume himself seems to be implying just that, in a closer examination it is too simple and unconvincing. He raises the question of whether to take the distinction between degree of force and vivacity as the determinative criterion for what makes impressions and ideas for what they are or as a mere symptom, helpful for distinguishing the two but not in any sense constitutive of what it is to be an impression or idea. Furthermore, Hume himself has said that in some particular instances ideas can be as vivid as impressions, and impressions as faint as ideas, a claim which raises an immediate question: Why do not ideas that approach to impressions in force and liveliness become impressions, and why do not impressions that cannot be distinguished from ideas become ideas? This question is enough to cast doubt on the interpretation which takes vivacity as the distinguishing criterion. Were a difference in degree of force and vivacity the only criterion, or simply the criterion, for distinguishing impressions and ideas there could never be any confusion or mistaking one for the other. The very fact that at times the two so resemble each other as to be indistinguishable, that attending solely to force and vivacity as the distinguishing criterion may lead us into error, should make it obvious that another criterion has to be found.

Landy argues that force and vivacity are best understood as “phenomenal symptoms” by which the distinction is recognised and best explained by the Copy Principle. What distinguishes impressions from ideas, in Landy’s opinion, is not force and vivacity but the Copy Principle: that ideas are copies of impressions, that impressions are original whereas ideas are derivative. For an idea to be a copy it must, however, fulfil two necessary and jointly sufficient conditions: the first condition is Causality in the sense Hume understands the term: that ideas are caused by their corresponding impressions and the second condition is Exact Resemblance: that ideas exactly resemble their corresponding impressions. If an entity, i.e., idea, does not meet these two conditions jointly then there is no copy. Landy maintains that “the best way to understand Hume’s distinction is whatever way, in accordance with the text, makes his philosophical position strongest” (p. 121). And in his view the Copy Principle makes Hume’s position stronger.

The substitute criterion for vivacity then is the Copy Principle. It states:

“That all our simple ideas in their first appearance are derived from simple impressions, which are correspondent to them, and which they exactly represent”

(T, p.52). Hume is clear: the Copy principle applies only to simple impressions and ideas, not to complex ones.

Landy intends to use the Copy Principle in a different way than the proponents of the first interpretation use it. Of the two interpretations I concur with Landy’s interpretation. The principle is usually presented as a claim about impressions and ideas. Landy presents it as a criterion, a theoretical explanatory principle for what it is to be an impression or idea. “It is because ideas are copies and impressions are not that the former are less forceful and vivacious than the letter, and it is that fact, in turn, that help us distinguish one kind of perception from the other introspectively as we do” (p. 129). There are however objections to this account which due to lack of space I cannot address in here.

What is the function of the application of this principle? There is one obvious answer: to dispel useless metaphysical subtleties that, according to Hume, have brought disgrace on philosophy: “When we entertain any suspicion that a philosophical term is employed without any meaning or idea (as is but too frequent) we need but inquire, from what impression is that supposed to be derived?” Hume, for example, would want to ask from what impression the idea of substance (or the idea of personal identity, or of space and time, of the idea of necessary connection between cause and effect, etc) is derived? There is no impression of space separate from the individual impression of the coloured points; all we can find is the impressions of coloured points “disposed in a certain order”. He believes that in cases like these we mistake words for ideas.

In support of his copy principle Hume offers two examples: when teaching a child the idea of colour blue you offer an actual blue object to produce the impression, not the other way round, and wherever there is a defect with the organs of impressions, their correspondent ideas are lost.

Hume treats the copy principle as an empirical generalization. However, immediately he suggests a counter example: THE MISSING SHADE OF BLUE. If a person is presented with a graded series of blues, running from the deepest to the lightest, and a particular shade of blue which she has never seen is absent, she will notice a blank in the continuous series and will be able to raise the idea of the particular shade. This suggests that some ideas are not derived or copied from impressions. So what is their origin?

Having devised this counter example, Hume wantonly dismisses it, saying that it is too ‘particular and singular’ to oblige to give up his general maxim, and continues to treat his principle as though it held universally granting it something like the status of an irrefutable a priori principle. So can the counterexample be explained away? For the Copy Principle to function as a criterion of the distinction we must find a way to accommodate the counterexample. After all, as we know from the history of science, it is not the case that every counterexample falsifies a scientific theory. The counter example must be so unlike an idea as to be a wholly different kind of mental entity to falsify the explanatory Copy Principle, which is not the case with the missing shade of blue. Hume thinks that the counter example is too particular as to be wholly different in kind.

Philosopher have, ever since, been puzzled, outraged even, by Hume’s brisk dismissal of the counterexample. Some have gone so far as to label such a dismissal ‘an affront to logic’, others as ‘scandalous’ and ‘unsettling’. But probably they have been arguing all too hastily. Probably Hume is right to dismiss the missing shade of blue counterexample. We however need to justify somehow Hume’s dismissal. John O. Nelson has argued that although Hume speaks of a contradictory phenomenon, he is speaking only of a conceivable contradictory phenomenon because the Copy Principle is not an a priory necessary proposition but a synthetic, a posteriori empirical proposition. In other word, the missing shade of blue projects merely the idea of a contradiction, not an actual exception that may falsify the principle. This is similar to Bertrand Russell’s line of argumentation, where he maintained that “Hume made himself an unnecessary difficulty in regard to the theory that images ‘copy’ impressions”, and went on to diffuse the counter example altogether arguing that “images are always more or less vague copies of impressions, so that an image might be regarded as a copy of any one number of different impressions of slightly different shades […] This is an example of an unreal puzzle manufactured by forgetting vagueness”. Hume’s exception to the Copy Principle, in Russell’s view, no longer remains an exception if we grant that ideas are as a rule not of one definite prototype but of a number of similar prototypes.

This is all very well, but does it actually represent correctly the Humean position? And how doe this fit with the second type of interpretation advanced by Landy. Russell and Nelson may want to find some textual evidence in Hume to support their claims. Landy says that the Copy principle “does have an empirical component, but it is not a mere empirical claim”. The principle does admit of the exception but it doesn’t undermine it. The validity of Landy’s interpretation cannot rest content with anything that Nelson and Russell have proposed, for that undermines the status of the Copy Principle as a criterion of distinction. Landy contrasts the missing shade of blue (M for short) with impressions of reflections in that “while not copied from mental entities, they are at least caused by other mental entities”. He wants to argue that M is like a complex idea that resembles in a way, or it is as good as, the impressions of reflexions. Because the idea of the missing shade of blue so resembles its antecedent and subsequent shades of blues, it is as good as copied. The resemblance is that close that we cannot speak of completely original idea which would undermine the Copy Principle and thereby delegitimize its grounding of the impression and idea distinction. This justificatory account is not completely satisfactory, but we cannot pursue it any longer.



Barry Stroud. Hume. Routledge, London and New York, 1981.

Bennett, Jonathan. Learning from Six Philosophers: Descartes, Spinoza, Leibniz, Locke, Berkeley, Hume. Volume 2. Clarendon Press, Oxford, 2001.

Garret, Don. Cognition and Commitment in Hume’s Philosophy. New York: Oxford University Press, 1997.

Hume, David. A treatise of Human Nature. Ed. Ernest C. Mossner, Penguin Books, 1985.

Hume, David. An Inquiry Concerning Human Understanding. Ed. Stephen Buckle, Cambridge University Press, 2007.

Landy, David. Hume’s Impression/Idea Distinction. Hume Studies, Volume 32, Number 1, April 2006.

Nelson, John O. Hume’s Missing Shade of Blue Re-viewed. Hume Studies Volume XV Number 2 November 1989.

Russell, Bertrand. An Outline of Philosophy. London: George Allen, 1949.

Smith, N.K. The Philosophy of David Hume. London: Macmillan, 1941. Reprinted: New York: Macmillan, St. Martin’s Press, 1960.


About albphilosopher

"The good life is one inspired by love and guided by knowledge." B. Russell
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One Response to Impressions and Ideas

  1. adventurist says:

    Nice essay,

    I’ve written about a few more criticisms of Hume on my blog here that you might find interesting; Let me know what you think of my responses to them. It would be good to hear some constructive criticism!

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