Bertrand Russell famously summarized Hume's contribution to philosophy, saying that he "developed to its logical conclusion the empiricist philosophy of Locke and Berkeley, and by making it self-consistent made it incredible." Hume is remarkable in that he does not shy away from conclusions that might seem unlikely or unreasonable. Ultimately, he concludes that we have no good reason to believe almost everything we believe about the world, but that this is not such a bad thing. Nature helps us to get by where reason lets us down.
Hume is unquestionably an empiricist philosopher, and he strives to bring the rigor of scientific methodology to bear on philosophical reasoning. His distinction between relations of ideas and matters of fact is absolutely crucial in this respect. Anything we can say about the world is a matter of fact, and thus can be justified only through experience and can be denied without contradiction. Relations of ideas can teach us about mathematical truths, but cannot, as some rationalist philosophers would have, teach us about the existence of our selves, an external world, or God.
If we are left with only matters of fact to get us by in the world, however, we find ourselves greatly limited. How can past experience teach me anything about the future? Even to infer without circularity that future experience will resemble past experience requires some principle that cannot be grounded in past experience. Without that principle, our ability to reason according to cause and effect, and thus the greater part of our ability to reason with matters of fact, is sharply curtailed.
We should be careful to note the tone Hume's skepticism takes here, however. Rather than conclude that we cannot know anything about future events or the external world, he concludes that we are not rationally justifiedin believing the things we do. Hume does not deny that we make certain inferences based on causal reasoning, and indeed insists that we would be unable to live if we didn't do so. His point is simply that we are mistaken if we think that these inferences are in any way justified by reason. That is, there are no grounds for certainty or proof of these inferences.
Hume is a naturalist because he suggests that nature, and not reason, leads us to believe the things we do. Habit has taught us that we are safe in making certain inferences and believing certain things, and so we don't normally worry about them too much. We cannot prove that there is a world external to our senses, but it seems to be a relatively safe assumption by which to live. Rather than try to justify our beliefs or identify the truth, Hume seeks simply to explain why we believe what we believe.
The Enquiry is decidedly a book about epistemology and not about metaphysics. That is, Hume is concerned about what and how we know, and not at all about what is actually the case. For instance, he does not deal with the question of whether there actually are necessary connections between events, he simply asserts that we cannot perceive them. Or perhaps more accurately, Hume argues that, because we cannot perceive necessary connections between events, the question of whether or not they actually exist is irrelevant and meaningless.
Hume is an ardent opponent of rationalist metaphysics, which seeks to answer questions such as whether or not God exists, what the nature or matter and soul is, or whether the soul is immortal. The mind, according to Hume, is not a truth-tracking device, and we misuse it if we think it can bring us to metaphysical conclusions. A Humean science of the mind can describe how the mind works and why it reaches the conclusions it does, but it cannot take us beyond the confines of our own, natural, reason.
Hume's stated method is scientific, of careful observation and inference from particular instances to general principles. The drive of scientific inquiry is to dig deeper and deeper so as to uncover a very few, very simple principles that govern all the complexities that we observe. Newton's genius gives us three very simple laws that can explain and predict all physical phenomena. Hume wishes to perform a similar feat for human understanding (the word "understanding" is used by Hume to describe most broadly the several faculties of human reason). The hope is that Hume will derive a similarly small and simple number of principles that can explain and predict the processes of human thought. His method will be to proceed from simple observation of how the mind works and how we use it in everyday life, and to infer from his observations increasingly general principles that govern our understanding until he reaches a bedrock of simplicity and clarity.
In this respect, Hume follows very much in the empiricist vein of philosophy and owes a large debt to “John Locke”. Locke moved against rationalist philosophy, best exemplified by “Descartes”, which relies heavily upon rational intuition. The empiricist tradition asserts that experience, and not reason, should serve as the basis of philosophical reasoning.
The motivation for Hume's project is made apparent in his complaint that the "accurate and abstract" metaphysics that he is pursuing is frequently looked down upon and disdained. The difficulty and counter-intuitive nature of these inquiries often lead to errors that may seem absurd and prejudicial to future generations. Even today, there is a great deal of debate as to whether there has been any real "progress" in philosophy: we may have refined our discussions and dismissed some bad ideas, but in essence we are still mulling over the same problems that concerned Plato and Aristotle. It would not be unreasonable to suggest that we are no nearer a satisfactory and final answer than the ancient Greeks. Hume hopes that scientific observation can uncover the principles that underlie our reasoning so that we can be more immediately aware of faulty logic and more easily guided along the correct path.
Ever since the scientific revolution of Newton, Galileo, and others, science has been held up as a paradigm of fruitful reasoning. In science, there is a carefully defined methodology that precisely details how we can test a theory and determine whether it is right or wrong. Though it is often difficult to determine the right answer, the scientific method usually prevents us from arriving at answers that are far from the mark. Philosophy lacks any such determinate method, and philosophers are continually taking up conflicting views. For instance, Hume's emphasis on observation goes directly against Descartes' rationalism, which disparages observation in favor of pure reason. Hume hopes that his empiricism will open the way for a carefully defined method that will not allow for such disparity amongst philosophers.
Hume also suggests that his work must be epistemically (epistemic: of, relating to, or involving knowledge; cognitive) prior to the new science that he so lauds. The scientific method is a product of careful reasoning, and is thus subject to the laws of human understanding. While science seems to be in far better shape than philosophy, it too can benefit from his work. In this way, Hume differs from his predecessor, Locke. Locke sees himself as laboring on behalf of the new science, clearing away some of the linguistic rubble that might lead to confusion. While Locke humbly sees himself as simply clearing a path for science, Hume believes that his own work must lay the groundwork upon which science can rest. If he can uncover the precise laws that govern our reasoning and inferences, this should help us draw the right conclusions in our scientific investigations.
Hume brings to bear three important distinctions. The first, and most important, is the distinction between ideas and impressions. This distinction is original to Hume and solves a number of difficulties encountered by Locke. A proper discussion of Hume's footnote would take us too far afield, but we should remark that Hume's criticism of Locke is exact and powerful. The distinction between impressions and ideas might seem quite obvious and of no great importance, but Hume is quite clever to identify the full importance of this distinction. An empirical philosophy asserts that all knowledge comes from experience. For Hume, this would suggest that all knowledge comes from impressions, and so ideas are set up as secondary to impressions.
The second distinction, between complex and simple impressions or ideas, helps draw out further the power of the first distinction. A simple impression might be seeing the color red, while a complex impression might be seeing the totality of what I see right now. A simple idea might be the memory of being angry while a complex idea might be the idea of a unicorn (composed of the idea of a horse and the idea of a horn). Complex ideas and impressions are compounded out of the simple ones.
With these first two distinctions, Hume is creating a hierarchy of mental phenomena. Since the complex is compounded out of the simple and ideas are derived from impressions, everything in our mind is based ultimately upon simple impressions. A complex idea is compounded out of several simple ideas, which are in turn derived from several corresponding simple impressions. Hume thus suggests that a term can only be meaningful if it can be connected with an idea that we can associate with some simple impressions. Hume, we should note, is silently implying that every term must be connected with some idea. In the eighteenth century the philosophy of language had not yet flourished, and it was not clear how difficult it might be to determine precisely how words, ideas, and reality link up. Hume's suggestion that all terms can be analyzed into simple impressions anticipates Russell, who argues that we can analyze all terms into simple demonstratives like "this" or "that." Hume's suggestion comprehends a picture of language according to which the words we use are a complex and opaque expression of a simpler underlying language which proper analysis can bring out.
The third distinction is the three laws of association. If the previous two distinctions give us a geography of the mind, describing its different faculties, this distinction gives us a dynamics of the mind, explaining its movement. According to Hume, any given thought is somehow related to adjacent thoughts just as any given movement in the physical world is somehow related to adjacent moving bodies. His three laws of association, then, might be seen as equivalent to Newton's three laws of motion. With them, Hume hopes to have described fully the dynamics of the mind.
There are a number of objections we might want to raise to Hume's distinctions and the way they are introduced, but we will touch on only a few briefly. First, we might ask how strictly we can distinguish between impressions. Hume argues that ideas can be vague, but that impressions are exact and that the boundaries between them are clearly defined. Is the boundary between the impression of a 57" stick and a 58" stick that clearly defined? There is some level of vagueness in our impressions that Hume does not acknowledge. We could also point out that while we are experienced in distinguishing colors, we are not so good with some other sensations. For instance, we often have trouble distinguishing between tastes.
Second, we might object to Hume's implicit philosophy of language. It seems closely linked to the idea that simple impressions are clearly defined and infallible. It is far from clear, however, why it should be desirable or possible to reduce all our language to simple impressions. What, we might ask, is the simple impression from which is derived the word "sake," for example?
Third, we might ask Hume to be clearer in his distinctions. For instance, are dream images impressions or ideas? Most likely they are ideas, since they consist of a mixture of imagination and memory. However, dreams are (arguably) phenomenally indistinguishable from waking experience: we cannot prove that we are dreaming from within a dream. Thus, all our impressions from within a dream are as real to us as we dream them as waking impressions are to us when we experience them.
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