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History of Philosophy in the 17th & 18th Centuries


Hume's picture of the mind



Hume's scientific perspective on understanding the mind

Hume sought to take forward the scientific view of the human being. As he saw it, Francis Bacon had begun the resurgence of science as applied to the natural world: he himself, (in company with 'Mr Locke, my Lord Shaftsbury, Dr Mandeville, Mr Hutchinson, Dr Butler, etc.') was to further 'the application of experimental philosophy to moral subjects' (Hume, Treatise, Intro, p.7).

Hume's mental 'corpuscularianism'

Hume, like the others, takes from the science of the natural world the notion that we should think of what we seek to understand as made up of simple discrete constituents. The natural world is to be thought of as made up of 'corpuscles': the mind is to be thought of as made up of 'mental' corpuscles, called by Locke 'ideas' but for which Hume most often uses the term 'perceptions'.

It is perhaps striking, from our own perspective, that Hume offers no explicit arguments in defence of this 'atomistic' conception - as applied either to the natural world or to the mind - but simply offers an outline, which would be succinct if it was not rather casual, which he clearly means to be understood as a reminder of what all his readers will take to be common ground. Fodor points out though that Hume's implicit argument is that the Theory of Ideas makes possible the construction of a successful psychology: the whole project of the Treatise. (Fodor, HV, p.8.)


The conception of the mind which Hume develops may not be all that inadequate. Jerry Fodor's little book Hume Variations blows a gale of fresh air through the widespread assumption that from our contemporary perspective we can easily see where, or at least that, the corpuscular theory of the mind (the "Theory of Ideas" ) goes badly wrong. More...

It is this conception of mentality which, so it is often argued, Hume's work in the end reveals as inadequate.

One key point at which the Theory breaks down, it is argued, is the account Hume tries to offer of what a human being is. He says a human being can only be thought of as a sequence of mental corpuscles: but can offer no explanation as to why a person mistakes such a sequence for a single thing enduring through time.

His inability to develop an explanation he thinks arises because he is committed to two principles which cannot both be true - cannot both be true, that is, if there is to be any explanation of how we mistake a sequence of ideas for a single thing which endures.

The first is his commitment to the Theory of Ideas. The second is his thesis, argued for with great rigour, that we can never observe any connection between Ideas. If both these things are true, and a person is nothing but a sequence of ideas, how can s/he possibly be misled into thinking the sequence which s/he truly is, is not a sequence at all but one enduring thing?

"In short," he says, "there are two principles, which I cannot render consistent; nor is it in my power to renounce either of them, viz, that all our distinct perceptions are distinct existences, and that the mind never perceives any real connexion among distinct existences. For my part I must ... confess that this difficulty is too hard for my understanding.' Treatise, Appendix, p.319.)


Some words in exposition of the Theory of Ideas Hume starts out with:

Hume's understanding of the difference between sense experience and memory/imagination (impressions v. ideas)

Hume tries to catch the difference between our seeing something and our thinking about it afterwards by saying that corpuscles differ in the 'degree of force and liveliness' with which they 'strike the mind'. If we think of sight as an example - a perennial temptation - there is a difference, we may agree, between the experience of actually seeing something and the experience of thinking about it afterwards, when for example we may speak of having an image of it 'before the mind's eye'. Hume's articulation of this is that in both cases the experience is to be thought of as there being a mental corpuscle in the mind, and that these two corpuscles are entirely similar. But in live sense experience the corpuscle appears in a forceful and lively - or 'vivacious' - way, whereas an exact copy of that corpuscle, appearing after the live experience has ended will appear exactly like the live-experience corpuscle except for seeming relatively 'faint''. Hume announces that he is going to call corpuscles of the first kind 'impressions' and the second 'ideas'.

Hume's word for 'corpuscle' in the above is 'perception'. His view is that 'all the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call impressions and ideas.' (Hume, Treatise, Book I, Part I, Section I, First sentence, p.11.)

The distinction between simple and complex perceptions

Some ideas are simple, some are complex, and impressions similarly. A complex in either case is made up ultimately of simples. A simple is 'such as to admit of no distinction or separation'. (Hume, Treatise, Book I, Part I, Section I, p.12)

This notion of a 'simple' perception is a foundational commitment of Hume's. It is in fact his commitment to the corpuscular model of the mind. It corresponds to the tenet that an atom is conceptually indivisible.

The closeness of the parallel as Hume sees it between physical corpuscles - the corpuscles of the Corpuscularianism - and perceptions as he understands them comes out clearly in his argumentation on space. The mind is finite, he takes it, and therefore cannot accommodate an infinite number of ideas.

"The capacity of the mind is not infinite; consequently no idea of extension or duration consists of an infinite number of parts or inferior ideas, but of a finite number, and these simple and indivisible..." (Hume, Treatise, Book I, Part II, Section IV)

What then is a 'simple' perception?

The key seems to be that a simple perception 'admits of no distinction or separation'.

The thought he is trying to make something of is this. If I consider the idea I have of an apple I discover that I can think of it as made up of a number of other ideas - for example, the idea of its colour, the idea of its taste, the idea of its smell. But if I go on to consider one of these component ideas - my idea of 'green' , it may be, if I had the idea of a green apple - I may discover that I am not in this case able to think of it as made up of components. Hume calls the sort of ideas which we cannot think of as made up of other ideas 'simple' and the others 'complex'.

The same distinction can be applied he thinks to impressions. So it is something that holds of perceptions in general he thinks - all perceptions come in two sorts, simple and complex.

I have presented a 'simple' perception as one which 'we cannot think of as' made up of a plurality of other perceptions. But Hume makes it clear that this incapacity of ours to think of a simple idea as made up of a plurality of 'components' is a consequence of the nature of simple ideas themselves: they are such as not to support such analysis. In other words, Hume takes it to be an objective fact about some ideas that they are not made up of other ideas, and that it is this which thwarts us when we try and 'split them up' in thought. 'Simple perceptions, or impressions and ideas, are such as admit of no distinction nor separation.' (Hume, Treatise, Book I, Part I, Section I, p.12, my italics)

But then we are left with the question: how do we know this is so? How do we know that some ideas 'admit of no distinction nor separation' and others do? Hume can I think only answer with the thought experiment which I have tried to reproduce above: some ideas are such that when you try and think of them as made up of other ideas you fail.

You may think Hume's position here is not very satisfactory, because it rules out for no reason at all the possibility that we might fail in such a bid not because the idea we are attempting to analyse is unsplittable but because through some inadequacy on our part we fail to split it.

You may not be wrong.

Impressions cause ideas, not the other way round

In what I have said I have already implied that ideas are caused by impressions - I look at an apple and get an impression thereof, but when I think about the apple afterwards what I have before my mind is an 'idea ' of it. Hume's thesis, we may quickly assume, is that the idea has been brought into existence by the impression. This is in fact Hume's view, but he is careful to acknowledge that it is a thesis that needs evidential support.

He provides this by first reporting on the results of 'the most accurate examination of which I am capable' (Hume, Treatise, Book I, Part I, Section I, p.13), which involves presumably a review of such ideas as he has, together with reflection on his perceptual experience. What is revealed he says is that "every simple idea has a simple impression, which resembles it, and every simple impression a correspondent idea." And in fact whenever a simple impression occurs, so does the idea which is 'correspondant' to it. He then appeals to the results of a 'second review' which establishes that whenever I have a simple idea for the first time, it is always preceded by its correspondant simple impression. This is enough to establish that it is simple impressions that cause ideas and not the other way around.

'That I may know on which side this dependence lies, 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.'

He concludes:

'That all our simple ideas in their first appearance are derived from simple impressions, which are correspondent to them, and which they exactly represent.' (Hume, Treatise, Book I, Part I, Section I, p.14)

Hume and materialism

"Now as all objects which are not contrary are susceptible to constant conjunction, and as no real objects are contrary; it follows that, for ought we can determine by the mere ideas, anything may be the cause or effect of anything; which evidently gives the advantage to the materialists above their antagonists." (Hume, Treatise, Book I, Part IV, Section V, p.237)

Hume's understanding of belief

The corpuscular model of the mind suggests a way of thinking about belief. What is the difference, Hume asks, between merely having the idea of something and believing that thing exists? Both cases involve us in having the idea of the thing in question. And it must be exactly the same idea in both cases. The only difference, Hume thinks, must be in the way we have this idea. 'An idea assented to feels different from a fictitious idea... and this different feeling I endeavour to explain by calling it a superior force, or vivacity, or solidity, or firmness, or steadiness.' Hume, Treatise, Book I, Part III, Section VII, p.99.


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