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Hume: induction, causation and his theory of ideas Hume believed firmly that vast improvements in natural science would occur if we could have knowledge of the 'extent and force of human understanding': getting to grips with the limits of our understanding might help to better focus our studies of the sciences. If the science of man is to be the foundation of all the other sciences, what is its foundation? Hume asserts that the science of man is founded upon 'experience and observation'. Like Locke, Hume believes the deeper causes of gravity, cohesion of matter and the communication of motion (cause and effect!) to be 'shut up from human curiosity and enquiry'. The same approach applies to enquiry into the 'moral science' of man - further enquiry into causes anterior to the effects we experience is an 'intemperate desire', and these effects are best left as 'original qualities of human nature'. Hume is as a rule reluctant to involve himself in consideration of questions that reach beyond the immediate facts of experience - the effects of sensory impressions and ideas are caused by 'unknown causes', which are 'perfectly inexplicable to human reason'. Hume divides the objects of human reason into 'relations of ideas' and 'matters of fact', in a similar distinction to Locke's 'knowledge' and 'belief' (or 'experimental knowledge'). Whilst Hume naturally has no problem with our believing that we know the truth of relations of ideas, he goes on to ask what evidence, or evidence of what nature, can assure us of the truth of matters of fact? Why do we hold the belief that the sun will rise tomorrow, and not its equally intelligible opposite, that it will not?
Hume's theory of ideas Where Locke has the single word 'idea', Hume employs three: for him the genus of items that Locke labels 'ideas' are perceptions, and within this genus lie two species of perception, ideas and impressions. Perception, for Hume, covers the 'whole range of mental events' (Bennett). Impressions = 'all our sensations, passions and emotions'. Ideas = the 'faint images' of these impressions in reason, thought and memory. Thus the difference between the experience of pain and the memory of that pain is not a difference in kind, but only in degree - ideas are simply less vivid copies of impressions. This is Hume's copy principle. The division between impressions and ideas thus posits that impressions are strong, lively, vivacious and intense, while ideas are weak, faded, faint and languid. Impressions are what we acquire from sense experience, whilst ideas are the raw materials of thought, understanding and comprehension. Hume believes that these two means of distinguishing impressions and ideas actually run into one another: impressions are strong and vivacious since they impact upon the mind with intensity and the immediacy of experience, whilst ideas well up inside the mind, as 'faint images of these [impressions] in thinking and reasoning'. In fact, however, the two distinctions Hume draws (strength of mental impact and internal/external starting point) come nowhere near to coinciding, and Hume's assumption that thinking about a particular state of affairs is akin to 'feebly experiencing' it seems manifestly false. Despite the unlikely nature of Hume's understanding of ideas and impressions, he uses it for the basis as his meaning-empiricism, which asserts that the limits on what one can understand are set by the limits of what one has experienced. Hume makes the same distinction as Locke and Gassendi between complex and simple ideas, such that we can imagine things we have never experienced, so long as we have experienced each constituent facet of that idea. A problem for Hume's theory of ideas: is a memory of being angry, or being in pain, really just being mildly angry, or in a small amount of pain?
Hume's 'meaning-empiricism' Hume adopts Locke's distinction between 'simple' and 'complex' ideas (simple ideas come from the senses, and can be amalgamated to form complex ideas) - the latter of which are linked with definable words, and the former of which are linked with words whose meanings must be learnt through experiential confrontation with examples. Hume's two arguments in defence of his meaning-empiricism:
1. everybody who has a given simple idea has at least one impression that 'corresponds' to that idea - that is, which resemble it in everything but strength. This, Hume argues, cannot be coincidence. Further, impressions must cause ideas, and not vice versa, since the idea never comes before the experience of that which the idea is about.
2. if a person is blind, such that they cannot acquire impressions of colours, they will have no corresponding ideas of particular colours (this is perhaps more simply a specific example of the first argument). Hume defends this argument by reference to the question of what could count as evidence against it. What would be required, Hume asserts, is evidence of a simple idea free from any corresponding impression - the 'production' of an idea that is not derived from an impression-source, but from some other. The theory of meaningempiricism, then, stands or falls on whether anyone can show or produce a simple idea not preceded by a correspondent impression. But what would constitute this evidence?
'Producing' an idea: to produce an idea is not merely to say that one has it, but to show that one has it - a congenitally blind man, then, must be able to give us reasons to believe him when he says 'I have an idea of purple'. Whilst the blind man may not be lying, Hume is confident that he will in fact not have any idea what purple means. So one can only demonstrate that one has an idea of something if one can demonstrate that one knows the meaning of it, or an accurate synonym of that thing. So someone counts as having an idea of purple iff he understands 'purple' or a synonym thereof, such that he can explain the concept. This seems to change Hume's theory somewhat, however, from a theory of ideas to a theory of understanding - it is not that ideas pre-require impressions, but that understanding pre-requires impressions. Hume's theory seems to work like this, then: (1) ideas follow impressions; (2) understanding is 'having ideas'; (3) 'understanding follows impressions'. For Hume, what we mean - what we 'are talking about' - when we employ a word or an idea, is strictly a function of that word or idea's impression-source.
Hume on causation It is very likely that Hume did believe in the existence of a necessary connection, but: 1) he asserted that it was ontologically unknowable. 2) he asserted that it was impossible to assert that necessary connection definitely did or didn't exist (he was a noncommittal sceptic) 3) he believed that, regardless of whether such a necessary connection existed or not, the idea we have of this necessary connection does not and cannot have come from the connection itself, because of 1). 'Hidden causal power': 'that power, by which one object-involving event stands in causal conjunction with another'. Although it is difficult to accurately define what this connection might consist of, since Hume, a non-committal sceptic with regard to external-world objects, argues that this power is by definition unknowable directly to the human mind, we can ascertain some details of its nature. Hume considers that this 'power': 1) cannot be observed in single instances, like qualities such as extension or shape may be, since causal power of this nature could only reveal itself in regularities and in the consistency of reaction to a particular action - that objectinvolving event B consistently follows object-involving event A. 2) should always be referred to in singular terms, as some overarching feature common by necessity to all objects, but at the same time, this power is not something complete in itself, manifest completely in single episodes of experience, but only through the repeated observation of consistent causal relations.
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