PHIL 2100: Introduction to Philosophy
Dr. Robert Lane
Lecture Notes: Monday June 21, 2010


[4.3.] David Hume and the Problem of Induction.


As we have seen, Descartes was concerned to overcome skepticism by showing that we can have knowledge of the external world.


In the material that we will read and discuss, David Hume is not primarily concerned with knowledge of the external world, but with justified belief in generalizations and predictions. As we will see, Hume argued in favor of a form of skepticism regarding that kind of justified belief.



[4.3.1.] Biographical Background.


·         1711-1776

·         Born in Edinburgh, Scotland; attended Edinburgh University.

·         At age 18, Hume fell so deep into his philosophical studies that he became ill.

·         At age 23, he became depressed, and left Scotland for Bristol, England, to go into business. He did not find that satisfying, so he eventually went to France to continue his philosophical studies.

·         While in France he wrote Treatise on Human Nature (I & II, 1739; III, 1740). It was a critical bomb.

·         He later wrote a popularized version, An Enquiry Concerning Human Understanding (1748). (excerpted at pp.246-253 of your textbook)

·         Died of bowel cancer in 1776.



[4.3.2.] General Points about Hume's Project.


1.      Hume belongs instead to the tradition of empiricism


empiricism (df.): the epistemological view according to which knowledge, or at least the most important knowledge, is to be gained only through experience. Empiricists reject substantive claims about the world that cannot be justified by appeals to experience (e.g. John Locke, George Berkeley, and Hume).


and is thus opposed to rationalism:


rationalism (df.): the epistemological view according to which the only, or at least the most important, source of knowledge about the world is reason, not the senses. [Prominent rationalists include René Descartes, Baruch Spinoza, and Gottfried Leibniz.]


On Hume’s view, the rationalist method used by Descartes—which relies solely on reasoning and not at all on the senses—cannot provide justification for any substantial, interesting claims about the world. (We can come to know trivial, a priori truths through reasoning, like “All bachelors are unmarried”; but we cannot discover anything of true interest that way.)


2. Hume argues that some important beliefs about the world cannot be justified either by reason or by experience. These include beliefs about the future; beliefs in the form of universal generalizations; and the belief that causality (the relationship of cause-and-effect) is real. As we will see, Hume defends a form of skepticism about such beliefs.


3. We have these important beliefs, not because they are supported by evidence, but because the human mind works in such a way that we cannot help but to believe them. Such beliefs cannot be given a philosophical justification, but they can be given a psychological explanation—an explanation of why we believe them, even if we are never justified in doing so.



[4.3.3.] Knowledge vs. Justified Belief.


As a first approach to our examination of Hume’s skepticism, first notice that knowledge is not the same thing as justified belief. It is possible to be justified in one’s belief that p without knowing that p.[1] Even if we do not know as much as we think we do—for example, even if Descartes is wrong and we never have knowledge of the world outside our own minds—it is still possible that many of our beliefs are rational or well justified. In other words, even if we lack knowledge, we might still have good reasons for our beliefs.


This means that the issue of whether we have knowledge is independent of the issue whether we have justified beliefs. Even if it turns out we do not know anything (or much of anything), this may be no big deal, since it does not imply that most of our beliefs are unjustified. It is possible to accept skepticism about knowledge but still accept that many of our beliefs are well-supported by the evidence:


                Although skepticism about knowledge sounds like a shocking thesis… [see the remainder of this passage, Sober pp.181-82]



[4.3.4.] Hume’s Skepticism about Justified Belief.


The kind of skepticism that Hume defends is a stronger form of skepticism than what we have examined up to now. It is not aimed merely against the possibility of knowledge; it is aimed against the possibility of specific kinds of justified belief.


However, there is another way in which Hume’s skepticism is more limited: it is applied, not to claims about the external world as it is here and now, but to two sorts of beliefs that go beyond telling us how the world is here and now:


Descartes’ anti-skeptical claim

Hume’s skeptical claim

we can have justified (true) beliefs about the external world as it is here and now


we cannot have justified beliefs that are about the future or are universal generalizations.



To illustrate the difference between the two claims, consider these three types of belief:


1: beliefs about present experience




“It seems to me that there is a black crow in front of me right now.”

2: beliefs about the external world here and now




“There is a black crow in front of me right now.”

3: beliefs about the future, or generalizations




“The next crow I see will be black” or “All crows are black”



Like Descartes, Hume is a sort of foundationalist…


Descartes tried to show that level 1 beliefs can justify level 2 beliefs.


Hume will try to show that level 2 beliefs cannot justify level 3 beliefs.


There are two assumptions of Hume’s (and Descartes’) foundationalism:

·         if level 2 beliefs are justified at all, they are justified by level 1 beliefs;

·         if level 3 beliefs are justified at all, they are justified by level 2 beliefs;

These assumptions reflect the idea that the “flow of justification” always goes up. There is no question whether level 3 beliefs justify level 2 beliefs, etc.:














Hume’s belief that level 2 beliefs cannot justify level 3 beliefs amount to the following: We cannot construct logically good arguments with all level 2 premises and a level 3 conclusion.



[4.3.5.] “Hume's Fork.”


Hume famously divides all objects of inquiry into two exclusive and exhaustive types (“exclusive” means that they don’t overlap; “exhaustive” means that these are the only two). This distinction is famously known as Hume’s Fork.


The distinction is between relations of ideas and matters of fact. Anything that a human being is capable of knowing is either a relation between the ideas in her own mind or some matter of fact


All the objects of human reason or enquiry may naturally be divided into two kinds, to wit, Relations of Ideas, and Matters of Fact. Of the first kind are the sciences of Geometry, Algebra, and Arithmetic; and in short, every affirmation which is either intuitively or demonstratively certain. That the square of the hypotenuse is equal to the square of the two sides, is a proposition which expresses a relation between these figures. That three times five is equal to the half of thirty expresses a relation between these numbers. Propositions of this kind are discoverable by the mere operation of thought, without dependence on what is anywhere existent in the universe. Though there never were a circle or triangle in nature, the truths demonstrated by Euclid would for ever retain their certainty and evidence.

Matters of fact, which are the second objects of human reason, are not ascertained in the same manner; nor is our evidence of their truth, however great, of a like nature with the foregoing. The contrary of every matter of fact is still possible; because it can never imply a contradiction, and is conceived by the mind with the same facility and distinctness, as if ever so conformable to reality. That the sun will not rise to-morrow is no less intelligible a proposition, and implies no more contradiction than the affirmation, that it will rise. We should in vain, therefore, attempt to demonstrate its falsehood. Were it demonstratively false, it would imply a contradiction, and could never be distinctly conceived by the mind. (Enq sec. IV part I; Sober 246)



knowledge of

relations of ideas

knowledge of

matters of fact

a priori

(“discoverable by the mere operation of thought”)

a posteriori


(“without dependence on what is anywhere existent in the universe”)

contingent, i.e., possible but not necessary

(“The contrary of every matter of fact is still possible”)

(epistemically) certain

probable at best

examples: mathematics

“a2 + b2 = c2

 “(3 x 5) = .5(30)”

examples: natural science, history, commonsense empirical knowledge

“The sun will rise tomorrow.”

arrived at by “demonstrative reasoning” (i.e., valid deduction) or by “intuition”

arrived at by “the present testimony of the senses or the records of our memory” … or by something else…


Not all matters of fact are arrived at by the present testimony of our senses or by our memories. There are matters of fact that are not supported in either way, including

·         universal generalizations: statements that attribute some trait or property to every member of a given group (“all crows are black,” “the sun rises every day”)

·         predictions, i.e., claims about the future (“the next crow I see will be black,” “the sun will rise tomorrow”)


At this point, Hume asks: how are these kinds of claim supported?


It may, therefore, be a subject worthy of curiosity, to enquire what is the nature of that evidence which assures us of any real existence and matter of fact, beyond the present testimony of our senses, or the records of our memory. (Enq IV, sec.I; Sober 246)

[4.3.6] Cause and Effect.


On Hume’s view, if it is a matter of fact (not a relation between ideas) and you did not experience it yourself, then your knowledge is the result of reasoning from effect to cause.


All reasonings concerning matter of fact seem to be founded on the relation of cause and effect. By means of that relation alone we can go beyond the evidence of our memory and senses. If you were to ask a man, why he believes any matter of fact, which is absent; for instance, that his friend is in the country, or in France; he would give you a reason; and this reason would be some other fact; as a letter received from him, or the knowledge of his former resolutions and promises. A man finding a watch or any other machine in a desert island, would conclude that there had once been men in that island. All our reasonings concerning fact are of the same nature. And here it is constantly supposed that there is a connexion between the present fact and that which is inferred from it. Were there nothing to bind them together, the inference would be entirely precarious. The hearing of an articulate voice and rational discourse in the dark assures us of the presence of some person: Why? because these are the effects of the human make and fabric, and closely connected with it. If we anatomize all the other reasonings of this nature, we shall find that they are founded on the relation of cause and effect, and that this relation is either near or remote, direct or collateral. Heat and light are collateral effects of fire, and the one effect may justly be inferred from the other. (Enq. sec.IV Part I; Sober 247)


“Jim is in France right now.”





“My roommate made pasta for dinner.”

I infer this from the fact that I receive postcards from France, from the memory I have of his telling me that he is going to France and of me dropping him off at the airport ... What caused this postcard to arrive in my mailbox? What caused me to have the memory?


I infer this from the dishes I find in the sink, from the memory of having commanded him to prepare pasta last night... What caused those dishes? What caused that memory?



So all of the beliefs we have about matters of fact that we are not presently experiencing, and of which we have no memory of directly experiencing, rely on assumptions that we make about cause and effect.


So, if any of those beliefs is justified, our belief about cause and effect must also be justified:


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. (Enq. IV, sec. I; Sober 247)



[4.3.7.] Beliefs About Cause and Effect A Posteriori, Not A Priori.


Now Hume asks how we arrive at our belief that one thing causes another. It is not a priori, i.e., we cannot come to have those beliefs by reason alone.


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 reasoning 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 most sensible qualities, to discover any of its causes or effects. (Enq. sec.IV, part I; Sober 247)


     Imagine a very common cause and effect relationship, e.g., that being under water for an hour causes a typical person to die, or that falling into a fire will burn you. Hume claims that Adam, right after the moment of creation, would not be able to tell just from seeing and touching water that it can drown him, or from seeing and feeling the heat of fire that it would burn him. Nor, if he and Eve were to play a game of pool, would he be able to predict that when a cue ball hits another that the other ball will move. (Enq. sec.IV, part I; Sober 247) Hume concludes:


No object ever discovers, by the qualities which appear to the senses, either the causes which produced it, or the effects which will arise from it; nor can our reason, unassisted by experience, ever draw any inference concerning real existence and matter of fact. (Enq. sec.IV, part.I; Sober 247)


     Further, Adam will only come to know that being underwater causes death (or that falling into the fire causes one to be burned, or that being hit by a cue ball causes billiards to move) once he has seen the two events conjoined over and over again


Adam could not know these things a priori because there is no “logical connection” between a cause and its effect; we can never, just by thinking about an event, derive a conclusion regarding what its effects will be, in the same way that we can think about a triangle and derive that its interior angles add up to 180.  That A causes B is never a relation between ideas, known a priori.


                In a word, then, every effect is a distinct event from its cause. It could not, therefore, be discovered in the cause, and the first invention or conception of it, a priori, must be entirely arbitrary.  And even after it is suggested, the conjunction of it with the cause must appear equally arbitrary, since there are always many other effects which, to reason, must seem fully as consistent and natural.  In vain, therefore, should we pretend to determine any single event or infer any cause or effect without the assistance of observation and experience. (Enq. sec.IV part I; Sober 248)


So, since our beliefs about cause and effect are not a priori, they must be a posteriori—a result of experience rather than of reasoning.



[4.3.8.] What is Our Experience of Cause and Effect?


But think about what we actually experience—what we see, hear, etc.—when we “experience” one event causing another. Hume points out that we never actually experience the causation.


It must certainly be allowed, that nature has kept us at a great distance from all her secrets, and has afforded us only the knowledge of a few superficial qualities of objects; while she conceals from us those powers and principles on which the influence of those objects entirely depends. Our senses inform us of the colour, weight, and consistence of bread; but neither sense nor reason can ever inform us of those qualities which fit it for the nourishment and support of a human body. Sight or feeling conveys an idea of the actual motion of bodies; but as to that wonderful force or power, which would carry on a moving body for ever in a continued change of place, and which bodies never lose but by communicating it to others; of this we cannot form the most distant conception. (Enq. sec.IV part II; Sober 250)


Instead, all we experience is the constant conjunction of the event and the cause. For example, in our experience, when in the past we have eaten bread, it has nourished us, and this has happened over and over again. Based on that constant conjunction, we assume that the very same thing will happen the next time, and indeed every time, we eat bread:


But notwithstanding this ignorance of natural powers and principles, we always presume, when we see like sensible qualities, that they have like secret powers, and expect that effects, similar to those which we have experienced, will follow from them. If a body of like colour and consistence with that bread, which we have formerly eat, be presented to us, we make no scruple of repeating the experiment, and foresee, with certainty, like nourishment and support. Now this is a process of the mind or thought, of which I would willingly know the foundation. (Enq. sec.IV part II; Sober 250)



[4.3.9.] Arguments for Generalizations and Predictions.


At the end of that last quoted passage, Hume is asking: what is the basis for the following kinds of inference, i.e., how can any of the following arguments be justified?


Arguments with General Conclusions


                        I’ve eaten many pieces of bread, and each has been nourishing.

(GEN)              Hence, all bread is nourishing.


                        I’ve observed the sun rising each day that I have been alive.

(GEN)              Hence, the sun has risen and will rise every day.



Arguments with Conclusions about the Future


                        I’ve eaten many pieces of bread, and each has been nourishing.

(PRED)                        Hence, the next piece of bread that I eat will be nourishing.


                        I’ve observed the sun rising each day that I have been alive.

(PRED)                        Therefore, the sun will come up tomorrow.



We frequently do engage in this sort of inference… but why? We have never seen (or heard, or felt, etc.) cause and effect. We have only ever seen (etc.) a constant conjunction between different kinds of event. So why do we so frequently infer, from the fact that a conjunction has been observed in the past, that it will continue into the future?


[I]f you insist that the inference is made by a chain of reasoning, I desire you to produce that reasoning. The connexion between these propositions is not intuitive. There is required a medium, which may enable the mind to draw such an inference, if indeed it be drawn by reasoning and argument. What that medium is, I must confess, passes my comprehension; and it is incumbent on those to produce it, who assert that it really exists, and is the origin of all our conclusions concerning matter of fact. (Enq. sec.IV part II; Sober 251)



[4.3.10.] “Demonstrative Reasoning”: Are GEN and PRED Arguments Deductively Valid?


Now Hume distinguishes between two types of argument: demonstrative reasoning and moral reasoning.


·         “demonstrative reasoning”: reasoning about relations of ideas; this sort of reasoning is a priori and capable of being deductively valid.


Jim is a bachelor.

Therefore, Jim is unmarried.


This geometric figure is a square.

Therefore, this geometric figure has four sides.


He then points out that arguments for GEN and PRED conclusions cannot be demonstrative, i.e., they cannot be deductively valid


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. (Enq. sec.IV part II; Sober 251)


Even if every piece of bread we’ve encountered up to now has been nourishing, it is always possible that the next piece will not be. Even if every crow we’ve seen up to now has been black, it is always possible that the next one will not be.


Since validity require that it be impossible for the premise to be true and the conclusion false at the same time, these kinds of arguments from past experience to generalizations and predictions cannot be deductively valid.

In other words, we cannot construct deductively valid arguments with level 2 premises and level 3 conclusions.


I have observed many crows and all that I have seen up to now have been black.

Therefore, the next crow I see will be black.


Therefore, all crows are black.


This type of argument is invalid: it is possible for the premise to be true and the conclusion false.



[4.3.11.] “Moral Reasoning”: Are GEN and PRED Arguments Inductively Strong?


By “moral reasoning,” Hume does not mean reasoning about right and wrong, about morality and immorality; rather, he means

·         any reasoning “concerning matter of fact and existence”;

·         such arguments are not a priori; they always begin with a posteriori premises;

·         they are never deductively valid; at best, they are inductively strong.


Obviously, the GEN and PRED arguments are meant to be inductive.


induction (df.): an inductive argument is one that takes a description of some sample and extends that description to items outside the sample.


The double-line separating the premise from the conclusion of each argument indicates that the arguments are not supposed to be deductively valid (see Sober p.182, where he states the arguments about emeralds).


So when he considers “moral reasoning,” Hume is considering whether arguments for GEN and PRED conclusions are ever inductively strong.


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. (Enq. sec.IV part II; Sober 251)



Here Hume is claiming that we cannot construct logically strong inductive arguments with level 2 premises and level 3 conclusions. This is a very radical claim! The bread and sunrise arguments seem inductively strong. So why does Hume say this?


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. (Enq. sec.IV part II; Sober 251)


On his view, all inductive arguments have an implicit premise


implicit premise (df.): a premise that is not explicitly stated as part of the argument, but which is nonetheless being assumed by the person putting forward the argument.


Sober calls the implicit premise “the Principle of the Uniformity of Nature”:


The Principle of the Uniformity of Nature (PUN) (df.): Nature is uniform, such that the future will resemble the past.


So the crow argument really goes like this:


I have observed many crows and all that I have seen up to now have been black.

(Implicit) PUN: Nature is uniform, such that the future will resemble the past.

(GEN)  Therefore, all crows are black.


(PRED)            Therefore, the next crow I see will be black.


If this is a good argument, i.e., if the premises give us good reason to accept the conclusion, then each of the premises, including PUN, must itself be justified. But what reason do we have for believing PUN?


Hume says: we have no good reason to believe it!


He will now argue that it cannot be defended deductively or inductively.



[4.3.12] Deductive Arguments for PUN,


When a man says, I have found, in all past instances, such sensible qualities conjoined with such secret powers: And when he says, Similar sensible qualities will always be conjoined with similar secret powers, he is not guilty of a tautology, nor are these propositions in any respect the same. You say that the one proposition is an inference from the other. But you must confess that the inference is not intuitive; neither is it demonstrative… (Enq. sec.IV part II; Sober 252)


Hume is considering this deductive argument for PUN is as follows:


Nature has been uniform in my past observations

So, nature in general is uniform (PUN).


But this argument is not deductively valid, since observations of uniformity in the past don’t guarantee uniformity in the future.


Hume makes the point by saying that the person who uses this argument is “not guilty of a tautology”…


tautology (df.): a logically true statement, e.g., “all bachelors are unmarried,” “either I am taller than 5 feet or I am not,” any statement of the form “either it is the case that p or it is not the case that p.”


His point is something like this: the word “nature” doesn’t mean uniformity, so there is no contradiction in supposing that the future will not be like the past.


In other words, PUN is not an a priori truth. It is not contradictory to say that nature has been uniform in the past but will not be uniform in the future.


[4.3.13.] Inductive Arguments for PUN.


To say it is experimental [i.e., a a posteriori argument based on past experience], is begging the question. For all inferences from experience suppose, as their foundation, that the future will resemble the past, and that similar powers will be conjoined with similar sensible qualities. If there be any suspicion that the course of nature may change, and that the past may be no rule for the future, all experience becomes useless, and can give rise to no inference or conclusion. It is impossible, therefore, that any arguments from experience can prove this resemblance of the past to the future; since all these arguments are founded on the supposition of that resemblance. Let the course of things be allowed hitherto ever so regular; that alone, without some new argument or inference, proves not that, for the future, it will continue so. In vain do you pretend to have learned the nature of bodies from your past experience. Their secret nature, and consequently all their effects and influence, may change, without any change in their sensible qualities. (Enq. sec.IV part II; Sober 252-53)


No argument for PUN is inductively strong.


Since all inductive arguments assume PUN as a premise, any inductive argument for PUN begs the question:


Nature has been uniform in my past observations

(Implicit) PUN: Nature in general is uniform.

So, nature in general is uniform (PUN).


In other words, any inductive argument for PUN must covertly assume the very claim that it is setting out to prove, i.e., it would be a case of circular reasoning.



[4.3.14.] Hume’s Skeptical Argument.


Now we ready to see all of Hume’s Skeptical Argument (1 – 7 are the same as on Sober 184—in this statement of the argument, I am going a bit beyond Sober’s analysis):


  1. Every inductive argument requires PUN as a premise.
  2. If the conclusion of an inductive argument is rationally justified by the premises, then those premises must themselves be rationally justifiable.
  3. So, if the conclusion of an inductive argument is justified, there must be a rational justification for PUN. (1,2 – valid)
  4. If PUN is rationally justifiable, then there must be a good inductive argument or a good deductive argument for PUN.
  5. There is no good inductive argument for PUN, since any inductive argument for PUN will be circular.
  6. There cannot be a good deductive argument for PUN, since PUN is not a priori true, nor does PUN deductively follow from the observations we have made to date.
  7. So, PUN is not rationally justifiable. (4,5,6 – valid)
  8. So, the conclusion of an inductive argument is never rationally justified. (3,7 – valid)
  9. The only sort of argument that can be given in support of predictions and generalizations (level 3 beliefs) is an inductive argument from level 2 beliefs.
  10. So, the predictions and generalizations are never rationally justified. (8,9 – valid)


This is Hume’s argument for


Humean Skepticism (df.): the view that predictions and generalizations cannot be rationally justified. [This is what Hume’s view is frequently called, even though he declined to use the word “skepticism” for it—Enq. sec.IV part II; Sober 253]



[4.3.15.] Should We Stop Believing Generalizations and Predictions?


Hume himself did not stop eating bread, or believing that the sun will rise tomorrow. And he does not expect us to stop believing those sorts of things. The point of his skeptical argument is not to get us to give up our general and predictive beliefs—it is to reveal to us our ignorance.


My practice, you say, refutes my doubts. [I.e., what I actually do and believe when I’m not philosophizing is inconsistent with the position that I’m taking here.] But you mistake the purport of my question. As an agent, I am quite satisfied in the point; but as a philosopher, who has some share of curiosity, I will not say scepticism, I want to learn the foundation of this inference. No reading, no enquiry has yet been able to remove my difficulty, or give me satisfaction in a matter of such importance. Can I do better than propose the difficulty to the public, even though, perhaps, I have small hopes of obtaining a solution? We shall at least, by this means, be sensible of our ignorance, if we do not augment our knowledge. (Enq. sec.IV part II; Sober 253)




[4.3.16.] The Role of "Habit."


Hume insists that, when we jump to GEN and PRED conclusions from past experience, we do not do so by way of reasoning or inference:


It is certain that the most ignorant and stupid peasants— nay infants, nay even brute beasts—improve by experience, and learn the qualities of natural objects, by observing the effects which result from them. When a child has felt the sensation of pain from touching the flame of a candle, he will be careful not to put his hand near any candle; but will expect a similar effect from a cause which is similar in its sensible qualities and appearance. If you assert, therefore, that the understanding of the child is led into this conclusion by any process of argument or ratiocination, I may justly require you to produce that argument; nor have you any pretence to refuse so equitable a demand. You cannot say that the argument is abstruse, and may possibly escape your enquiry; since you confess that it is obvious to the capacity of a mere infant. If you hesitate, therefore, a moment, or if, after reflection, you produce any intricate or profound argument, you, in a manner, give up the question, and confess that it is not reasoning which engages us to suppose the past resembling the future, and to expect similar effects from causes which are, to appearance, similar. This is the proposition which I intended to enforce in the present section. If I be right, I pretend not to have made any mighty discovery. And if I be wrong, I must acknowledge myself to be indeed a very backward scholar; since I cannot now discover an argument which, it seems, was perfectly familiar to me long before I was out of my cradle. (Enq., sec.IV part II; Sober 253)


Rather, we do so because of custom or habit.


For wherever the repetition of any particular act or operation produces a propensity to renew the same act or operation, without being impelled by any reasoning or process of the understanding, we always say, that this propensity is the effect of Custom. By employing that word, we pretend not to have given the ultimate reason of such a propensity. We only point out a principle of human nature, which is universally acknowledged, and which is well known by its effects. Perhaps we can push our enquiries no farther, or pretend to give the cause of this cause; but must rest contented with it as the ultimate principle, which we can assign, of all our conclusions from experience… [A]fter the constant conjunction of two objects—heat and flame, for instance, weight and solidity— we are determined by custom alone to expect the one from the appearance of the other. This hypothesis seems even the only one which explains the difficulty, why we draw, from a thousand instances, an inference which we are not able to draw from one instance, that is, in no respect, different from them. Reason is incapable of any such variation. The conclusions which it draws from considering one circle are the same which it would form upon surveying all the circles in the universe. But no man, having seen only one body move after being impelled by another, could infer that every other body will move after a like impulse. All inferences from experience, therefore, are effects of custom, not of reasoning. (Enq., sec.V part I; not in Sober)



Our beliefs that events are related by cause and effect relationships are completely nonrational.  We have no good reason to accept them.  We believe them, but in virtue of a kind of natural instinct; when we experience the constant conjunction of events, we form a habit of expecting the second when we observe the first.


This, says Hume, explains why we come to believe that As cause Bs only after seeing several instances of an A followed by a B.  If we arrived at that belief via reasoning, then we would come to believe it after seeing only one such instance, just as a man knows exactly as much about circles after seeing one of them as he does after seeing a thousand of them.


Material appearing below this point in today’s notes will not be covered on tomorrow’s test; I include it only in case you are interested in thinking about alternatives to Descartes’ and Hume’s foundationalism.



[4.3.17.] Beyond Foundationalism.


The key to overcoming Humean Skepticism is to question the foundationalist approach Hume (and Descartes) took to questions of justification.  Remember the two assumptions of Hume’s (and Descartes’) foundationalism:















These assumptions reflect the idea that the “flow of justification” always goes up. There is no question whether level 3 beliefs justify level 2 beliefs or level 1 beliefs and no question whether level 2 beliefs justify level 1 beliefs.


Sober, and many other contemporary philosophers, reject foundationalism. Sober makes the following criticisms of Descartes’ and Hume’s forms of foundationalism…



[] Sober’s Criticism of Descartes’ Foundationalism.


·         Descartes was right in thinking that something besides first person psychological beliefs is needed to justify level 2 beliefs. This is because beliefs about the content of one’s own mind are not enough to justify beliefs about the external world. There must be some “bridge principle” between those two types of belief.


bridge principle (df.): a belief about the relationship between beliefs at different levels (either between level 1 and level 2 beliefs, or between level 2 and level 3 beliefs)


We naturally take the level 1 belief


a)      “I seem to see a dog in front of me.”


to be a good reason for believing the level 2 belief


b)      “There is a dog in front of me.”


because we make background assumptions like:


c)      “My environment is normal and my senses are working properly.”

d)      “I am not under the influence of post-hypnotic suggestion.”

e)      “I am not a brain in a vat being fed images of a dog by a supercomputer.”



Beliefs (c), (d) and (e) are background assumptions that serve as bridge principles and

increase the justification of (b). They are level 2 beliefs that cannot be justified solely by level 1 beliefs.


This illustrates a fact about beliefs about the external world, a fact that foundationalism seems to ignore: beliefs about the external world (level 2 beliefs) are often made more justified by other beliefs about the external world (level 2 beliefs) which cannot be justified by first-person psychological beliefs (level 1 beliefs) alone.


The foundationalist view (adopted by Descartes) that


            If a level 2 belief is justified, it is justified only by level 1 beliefs.


leads to skepticism about knowledge (or justified belief) about the external world, because level 1 beliefs by themselves cannot justify level 2 beliefs.


But this should not lead us into adopting skepticism about the external world, because Descartes’ foundationalist view is false.



[] Sober’s Criticism of Hume’s Foundationalism.


The foundationalist view adopted by Hume that


If a level 3 belief is justified, it is justified only by level 2 beliefs.


leads to skepticism about justified belief (or knowledge) in the form of predictions and generalizations. This is because level 2 beliefs by themselves cannot justify level 3 beliefs.


But this should not lead us to accept skepticism about generalizations and predictions, because Hume’s foundationalist assumption is false. Level 3 beliefs are often held with good reason, and those reasons can be on all three levels:






In other words, premise 9 of Hume’s Skeptical Argument (viz. “The only sort of argument that can be given in support of predictions and generalizations (level 3 beliefs) is an inductive argument from level 2 beliefs”) is false.



[] What About Inductive Inference?


But Sober’s approach might have rescued predictions and generalizations (by showing that such beliefs can be justified, even if we cannot construct inductively strong arguments for them based solely on level 2 premise), where does it leave induction?


Remember how Hume attempted to show that inductive arguments are never strong (line 1 through 8 of his argument). The problem that Sober identified in that argument was with premise 9. We haven’t yet identified any problems in Hume’s argument up to line 8. That means that it is possible that he has given a strong argument for this claim:


The conclusion of an inductive argument is never rationally justified.[2]



Stopping point for Monday June 21. No new reading for next time. During the first half of class we will have your second test. During the second half, we will begin discussing the philosophy of mind.


[1] One obvious difference between knowledge and justified belief is that knowledge requires truth but justified belief does not. There is another difference as well: since justification comes in degrees, it is possible to have a belief that is true and justified to some degree, but not justified enough to count as knowledge. Sober says that the impossibility of error is necessary for knowledge, but not for justified belief (181). Not everyone would agree with Sober about this. If you accept the traditional definition of knowledge as JTB (justified true belief), you can maintain that knowledge does not require the impossibility of error. (However, as we have seen, there are big problems with the JTB theory of knowledge.) Sober is here embracing a very strong conception of knowledge.

[2] Back in lecture 16 (which we skipped), Sober considered a newer version of Hume’s argument for this claim (p.191) which doesn’t involve PUN. Then he considers two criticisms of it (by Strawson and Black) and argues that those criticisms fail. Then he drops the subject.


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