David Hume for Novice Philosophers. Part I: Sceptical Empiricist and Philosophical Naturalist

David Hume

Bruce R Long B App Sc. (Computing), BA Hons 1 (Philosophy), MPhil (Eng.) PhD candidate (Analytic Philosophy) 

Although one of the most renowned of the British empiricist philosophers, David Hume is best known for his scepticism, the metaphysical and epistemic implications of which have been debated by philosophers since Kant (who opposed Hume's metaphysics and moral theories strenuously on behalf of Protestant patrons and their theist doctrine). The best and most famous example of Hume's scepticism is his view of causality.

Hume asserted that we could never be certain that a particular kind of physical effect would always follow from the same kind of physical cause. The next time you drop something, gravity might behave anomalously and the object might go sideways. This is referred to as the problem of induction:

But to convince us, that all the laws of nature, and all the operations of bodies without exception, are known only by experience, the following reflections may, perhaps, suffice. Were any object presented to us, and were we required to pronounce concerning the effect, which will result from it, without consulting past observation; after what manner, I beseech you, must the mind proceed in this operation? It must invent or imagine some event, which it ascribes to the object as its effect; and it is plain that this invention must be entirely arbitrary. The mind can never possibly find the effect in the supposed cause, by the most accurate scrutiny and examination. For the effect is totally different from the cause, and consequently can never be discovered in it. Motion in the second Billiard-ball is a quite distinct event from motion in the first; nor is there any thing in the one to suggest the smallest hint of the other. A stone or piece of metal raised into the air, and left without any support, immediately falls: But to consider the matter a priori, is there any thing we discover in this situation, which can beget the idea of a downward, rather than an upward, or any other motion, in the stone or metal?(Hume, D. Enquiry,  Section IV, http://www.davidhume.org/texts/ehu.html, E4.9 )

One could be forgiven for thinking that this means that Hume was a sceptic about nature and natural laws themselves as real, and therefore sceptical about the basis of science in natural lawful nomic (law like or invariant) constraints. However, his scepticism is about our own epistemic access to nature: what we can possibly know about it with certainty. Hume's experiential data-gathering empiricism is exemplified clearly in the above passage in the first and second sentences. He thought empirical data was the only way of knowing anything (any matters of fact, or contingent facts in the world) reliably. However, Hume's scepticism about our ability to really know what is going on in nature and in causality troubles this empiricist view:

If we would satisfy ourselves, therefore, concerning the nature of that evidence, which assures us of matters of fact, we must enquire how we arrive at the knowledge of cause and effect. I shall venture to affirm, as a general proposition, which admits of no exception, that the knowledge of this relation is not, in any instance, attained by reasonings a priori; but arises entirely from experience, when we find, that any particular objects are constantly conjoined with each other. Let an object be presented to a man of ever so strong natural reason and abilities; if that object be entirely new to him, he will not be able, by the most accurate examination of its sensible qualities, to discover any of its causes or effects.(Hume, D. Enquiry Section IV, http://www.davidhume.org/texts/ehu.html, E 4.6, Emphasis Added)

I suggest that to regard Hume as a sceptic about science and nature rather than an empiricist first would be a misapprehension. However, as we will see later in this post and in Part II, Hume’s scepticism presents some difficult problems for his naturalism, and for realism about the external natural world (where external means existing outside of and apart from the mind and thought). For now, however, we can observe that, in all of his other philosophical musings, Hume tended to either emphasise or else openly rely upon the explanatory the role of nature (via empirical internal or mental experience), and to openly eschew supernaturalism and supernaturalist testimony (belief in the existence of supernatural entities and the ability to know any such thing) as constituting any kind of explanatory option.

Nature and Hume

The term ‘nature’ was at the time of Hume’s writings a philosophical term of art, and its real meaning and correct definition have been debated by philosophers from Aristotle to John Stuart Mill (who famously attempted a disambiguation and lamented the difficulties associated with its use in his 1874 ‘On Nature’). This is especially with respect to conceptions of human nature. 

In Hume’s moral theory - which has come to be known as moral sentimentalism because of the central role of human sentiment as a basis for moral values - common moral sentiments are determined in part by internal psychological similarities between human agents (although the term 'psychological' is anachronistic to his career and work). These psychological similarities are rooted in both human nature, nature in a broader sense, and in shared culture. It is precisely for these reasons, and because of Hume's rejection of the role of anything supernatural, that Hume’s theory of moral sentiments is an ethical naturalist theory ³.

In Chapter 10 of his famous 'Enquiry Concerning Human Understanding'  (titled 'Of Miracles'), Hume elevates natural material evidence in conjunction with careful step by step consideration of empirical information above human testimony, regarding the latter as all but useless in comparison as a basis for reliable knowledge. This applies especially where a testimony claimed supernatural/miraculous, or highly unusual events that would be remarkable by natural standards.

What Hume rejected, along with certain a priori knowledge about things such as cause and effect and the reliability of testimony of miracles, was the kind of a priori certainty that Rene Descartes sought to identify in his cogito. The Cogito is the name for Descartes’most famous idea – I think therefore I am/exist (which he never in fact stated in so many words). It is the idea that the only thing a person can know with certainty is that their thoughts exist. Descartes matched this posit with the idea that the Biblical God character (whom he thought actually existed) inspired and sustained a priori knowledge as well as sustaining mathematical truths. The foundation of all of these conclusions was Descartes' doctrine or principle of clear and distinct ideas: that if one could conceive of something as a clear and distinct idea, then that thing must be real. This is a form of a priori reasoning, or reasoning that does not require any experience of external (not mind or language dependent) past events and outcomes.

Recall that Hume rejected the value of a-priori reasoning pertaining to knowledge of facts about the world, and he emphasised the role of contradiction:

That there are no demonstrative arguments in the case, seems evident; since it implies no contradiction, that the course of nature may change, and that an object, seemingly like those which we have experienced, may be attended with different or contrary effects. May I not clearly and distinctly conceive, that a body, falling from the clouds, and which, in all other respects, resembles snow, has yet the taste of salt or feeling of fire? Is there any more intelligible proposition than to affirm, that all the trees will flourish in December and January, and decay in May and June? Now whatever is intelligible, and can be distinctly conceived, implies no contradiction, and can never be proved false by any demonstrative argument or abstract reasoning a-priori. (Hume, D. Enquiry Section IV, http://www.davidhume.org/texts/ehu.html, E 4.18. Emphasis added.)

The term ‘demonstrative arguments’ in this passage refers to arguments from what does normally happen contingently in the world, rather than what somehow should happen according to a-priori reasoning just in our heads without any recording of outside events. Hume is being sceptical about our ability to be certain about natural laws, including constancy of cause and effect. He is saying that just because we have seen a certain outcome n times does not mean that we can be assured that the outcome will occur again the same way at the n+1 occurrence. This is even if it seems to be governed by natural lawful constraints and has always been totally consistent in the past. There is no demonstrative argument available that will prove the certainty of the outcome next time: no argument from reference to all or any of the past examples of a similar system, event, or situation.

In the above quotation, Hume intends his scepticism and empiricism to turn Descartes' conception of clear and distinct ideas - the foundation of The Cogito - against the conclusion that it is supposed to support. Descartes doubted everything about human perception and science, but not the certainty of awareness of and existence of one’s thoughts, which he took to be undeniably real for certain because of the intervention of a god being. Hume dispenses with even that certainty because of his sceptical empiricism, and with it the ability to know the god being if it exists. Overall, what Hume has to rely upon is a probable knowledge only. That is to say, we can know only that something is probable by empirical means:

 If we be, therefore, engaged by arguments to put trust in past experience, and make it the standard of our future judgment, these arguments must be probable only, or such as regard matter of fact and real existence, according to the division above mentioned. But that there is no argument of this kind, must appear, if our explication of that species of reasoning be admitted as solid and satisfactory. We have said, that all arguments concerning existence are founded on the relation of cause and effect; that our knowledge of that relation is derived entirely from experience; and that all our experimental conclusions proceed upon the supposition, that the future will be conformable to the past.  (Hume, D. Enquiry Section IV, http://www.davidhume.org/texts/ehu.html, E 4:19. Emphasis added.)

That supposition is, as I have described above, something that Hume asserts we must be sceptical about due to our ignorance. We don’t have a clue what really joins a cause to its effect, says Hume: we only know that one always comes after the other in proximity to it. For Hume, knowing things are probably the case or probably a fact is as good as it gets. His doubts about the reliability of the senses as a basis for gaining information from nature empirically (by past experience including repeated experiment) are not nearly as strict as those of Descartes, and his empiricism puts him at odds with Descartes about the value of empirically based science for gaining knowledge. Yet he rejects Descartes a-priori as unreliable, and and along with that Descartes’ Cogito becomes as only probable knowledge, and not certain:

It is only after a long course of uniform experiments in any kind, that we attain a firm reliance and security with regard to a particular event. Now where is that process of reasoning, which, from one instance, draws a conclusion, so different from that which it infers from a hundred instances, that are nowise different from that single one? This question I propose as much for the sake of information, as with an intention of raising difficulties. I cannot find, I cannot imagine any such reasoning. But I keep my mind still open to instruction, if any one will vouchsafe to bestow it on me.(Hume, D. Enquiry Section IV, http://www.davidhume.org/texts/ehu.html, E 4:20. Emphasis added.)

Hume relies on the regularity available through empirical observation, but only as probably predictive about future events. Hume's sceptical doctrine of causality is often taken to be not only about epistemic access - or our inability to get enough information about what will happen in a particular kind of system to predict what it will do in the future - but about actual unpredictability in physical systems. However, arguably, Hume's bite-the-bullet approach to causality - which saw him assert scepticism about empirical certainty in the constant regularity of causes and their effects - is not intended to undermine his empiricism.

Thus Hume is a sceptic and an empiricist. However, Hume’s scepticism causes him what is arguably a significant problem for his empiricism.

Rene Descartes

How, then, is empiricism and Hume's version of naturalistic scientism (a term anachronistic to Hume) prevail in the face of his scepticism about cause and effect, and his general scepticism? His empiricism relies on probable knowledge from experiment, which is far closer to scientific methodology and reasoning than is Descartes’Cogito.


As I have mentioned above, Hume rejects the thoroughgoing scepticism about the reliability of the senses that causes Descartes to reject experimental and materialist science as ineffectual. Descartes rejects the senses as reliable, and favours a-priori thought as the only way to know, and to know the god of the Bible, which character he took to exist. Descartes elevates a-priori reasoning - including that based upon mathematics and logic - as perfect and god-inspired. Materialist science, however, he regards as completely undermined by necessary doubts about the reliability of human senses as a way of knowing what is real in the world. According to Hume, however, sense impressions (which are in fact a complicated internal conception of the senses due to Hume’s scepticism) of empirical experiences are exactly where probable synthetic knowledge - the only kind available - must originate.

Descartes

Descartes was a devout theist and theologian, and when he referred to God he was making a reference in religious faith. Hume's numerous references to ‘God’, on the other hand, are famously deistic (he appoints no specific theological character to, nor claims a belief in the existence of any god being) and arguably tongue in cheek or else calculated to endorse critical thinking on the matter of how knowledge of the divine might be sustained. That or they are far more ambiguous in terms of ontological and doxastic (belief) commitments. Such references are arguably also fashioned after his unambiguous critical rejection of both religious enthusiasm and religious testimony.

Importantly, Hume intends his scepticism and empiricism to turn Descartes' conception of clear and distinct ideas - the foundation of The Cogito - against the conclusion that it is supposed to support: God exists because he can be conceived of as a clear and distinct idea and is therefore real. According to Hume, one can’t rely on human testimony about alleged miracles, and if there is any kind of god, you couldn’t possibly know such a being existed a-priori (and thus by faith).

 Whatever is may not be. No negation of a fact can involve a contradiction. The non-existence of any being, without exception, is as clear and distinct an idea as its existence...That the cube root of 64 is equal to the half of 10, is a false proposition, and can never be distinctly conceived. But that Caesar, or the angel Gabriel, or any being never existed, may be a false proposition, but still is perfectly conceivable, and implies no contradiction. (Hume, D. Enquiry Section XII Part 3, http://www.davidhume.org/texts/ehu.html, E 12.28)

This is the basis of what philosophers refer to as Hume's Fork. Hume regarded that there are two kinds of knowing: knowledge of the relations of ideas in a-priori reasoning, and knowledge of external mind-independent matters of fact in the world. The first kind of knowledge - a-priori analytic knowledge, is demonstrated in the above passage. It involves the identification of true and false statements on the basis of the presence or absence of contradiction: on a logical basis. The second kind of knowledge (see earlier quoted passage E 4:18 above) - called synthetic a-posteriori - requires experience of patterns and regularities in nature or in the external world.  

According to Cartesian a priori reasoning and the principle of clear and distinct ideas: it is every bit as likely that there is not any god being as that there is one. According to Hume there is no knowledge to be had in this - only in what has been empirically experienced through the senses - or more precisely - sense impressions.

It is these kinds of claims that were regarded as broadly heretical by the theologians of the time, and the same theologians and their allies troubled Hume’s career thereafter. In Part II we will explore the impact of Hume's scepticism on his naturalism and his theory of mind (there are serious problems for the senses and the existence of the external world), and the damage done to him by the religionist academy. 

1. Descartes, R. Meditations on First Philosophy, translated by John Cottingham, Cambridge: Cambridge University Press, 1996.

 ( See also The Philosophical Writings of Descartes, trans. John Cottingham, Robert Stoothoff, Dugald Murdoch and Anthony Kenny, Cambridge: Cambridge University Press, 3 vols. 1984-1991.)

2. Hume, D. An Enquiry concerning Human Understanding, edited by Tom L. Beauchamp, Oxford/New York: Oxford University Press, 1999. (See also <http://www.davidhume.org/texts/ehu.html>)

3. Hume, D. An Enquiry concerning the Principles of Morals, edited by Tom L. Beauchamp, Oxford/New York: Oxford University Press, 1998.

4. Hume, D. Treatise of Human Nature (Oxford, 1978), ed. L.A. Selby-Bigge and P.H. Nidditch.

5. Mill, J. S., Collected Works of John Stuart Mill, J. M. Robson (ed.), Toronto: University of Toronto Press, 1963ff.