Version 2, November 6, 2005
For all their contributions to the advancement of our understanding of knowledge, the modern epistemologists are best known for their struggles with skepticism. Their post-Aristotelian way of describing the acquisition of perceptual knowledge, what Locke called the "new way of ideas," caused them nothing but trouble. As a result, philosophers for the most part have given up on the project of validation altogether.
There are two ways in which our assent can be validated. The first is, so to speak, retail. We can ask whether an individual proposition is one to which a person in a situation should assent. The answer we give is based on an evaluation relative to some given standard or standards. The Stoics set up such a standard, to which the Academic skeptics raised the objection that no one could use it successfully.
The more interesting way of validating assent is (to continue the figure of speech) wholesale. We want to know whether the standards that we actually use on the retail level are ones that we should adopt. This issue was the focus of Pyrrhonian skepticism. The Pyrrhonians challenged virtually all the standards that had ever been proposed. More importantly, they gave a general argument that suggests that there is no hope in justifying any given criterion for determining that to which it is proper to assent.
As seen in a previous module, the modern epistemologists agreed that knowledge requires certainty, or freedom from doubt. Thus in the case of a given proposition, it can be asked whether it is subject to doubt. If a standard is invoked to show that we are, then the validational question becomes whether the standard itself is such that whatever meets it will be certain.
Most of the important early epistemologists, including Galileo and Hobbes, showed no interest in general skeptical arguments. Descartes, on the other hand, sensed a threat from the Pyrrhonian arguments resurrected by Montaigne and others. Rising to the challenge, he took it upon himself to vanquish skepticism once for all. He seems to have been motivated by a desire to remove any grounds for objection to his own scientific and metaphysical views. If he could establish their absolute certainty, it would be impossible for his opponents to mount any cogent objections against his view.
In the Discourse on Method and Meditations, Descartes described his procedure differently. His goal, he tells us, is to purge his own mind of all of its false opinions. To do so, he sought a means whereby he could regulate his assent so as successfully to avoid any error.
To achieve this end, he engaged in a process of "hyperbolic doubt." He would regard as being false anything about which he could doubt in the slightest. He did this not by challenging individual beliefs, but rather by raising objections to the standards which we use to regulate assent to them. If he could then find a standard that could not be challenged, a standard which produced immunity from doubt, he would try to use it to build up a new edifice of knowledge on secure foundations.
What was unique about his approach was that he raised the most extreme skeptical hypotheses he could think of and then sought to prove that such hypotheses are false, thus removing any grounds for doubt. These hypotheses went far beyond the examples of wax pomegranates and indistinguishable eggs raised by the Academic skeptics against the Stoics.
As we saw, Plato had noted the difficulties in distinguishing waking states from dreaming states. Descartes took the problem a step further by suggesting that our entire experience might be nothing more than a prolonged dream. On this hypothesis, one's stream of ideas may give the appearance of external bodies, but in fact there are no bodies at all, not even the body of the dreamer. This has come to be known as the "dream hypothesis." We will deal with this hypothesis below.
The possibility that he is generally deceived was the most extreme skeptical hypothesis Descartes raised. He proposed that he might have been made by God in such a way that he was deceived in all his beliefs. One variant of the hypothesis is that there is an evil demon who bends every effort to keep him deceived about the existence of a physical world, while in fact (on this hypothesis) there is none. Instead, the demon is feeding him with dreams that ensnare his judgment. This metaphysical version of the dream hypothesis has come to be known as the "demon hypothesis."
The goal of the demon hypothesis seems to be to cut Descartes off from all opinions that he might have formed from the use of his senses. These "pre-conceived" opinions are, on Descartes's view, the primary source of error in human judgment. Descartes expressed his strategy pretty clearly in the Synopsis of the Meditations.
In the First Meditation reasons ar provided which give us possible grounds for doubt about all things, especially material things, so long as we have no foundations for the sciences other than those which we have had up till now. Although the usefulness of such extensive doubt is not apparent at first sight, its greatest benefit lies in freeing us from all our preconceived opinions, and providing the easiest route by which the mind may be led away from the senses.
From the description of the dream hypothesis, it appears that rational intuition at least is not affected by it. There is, however, a more powerful hypothesis that threatens it: the possibility that he had been created by God so as always to be deceived "every time I add two and three or count the sides of a square, or in some even simpler matter" (Meditation I). This seems, at least at first, to be consistent with God's supreme goodness, since God allows him to be deceived sometimes.
Moreover, the case is even worse if he were created by some being less perfect than God, or that he came to exist by chance. The less powerful his original cause is, "the more likely it is that I am so imperfect as to be deceived all the time." The reason for this is that "deception and error seem to be imperfections."
Decartes recognized that he was only able to doubt his previous opinions when he had the various skeptical hypotheses in mind. When he was not thinking about them, his old opinions crept back in. He could fend off his opinions about the physical world by keeping in mind the demon hypothesis. But he did not propose to keep in mind the hypothesis that God had created him so as to be deceived: "I will suppose therefore that not God, who is supremely good and the source of truth, but rather some malicious demon of the utmost power and cunning has employed all his energies in order to deceive me" (Meditation I).
Since the demon can only fool him by presenting false images of things, Descartes is not constrained to doubt what comes before rational intuition. That is, "something very simple and straightforward in arithmetic or geometry, for example that two and three added together make five" (Meditation III). And in fact, he is unable to doubt it when he is thinking about it.
And yet when I turn to the things themselves which I think I perceive very clearly, I am so convinced by them that I spontaneously declare: let whoever can do so deceive me, he will never bring it . . . that two and three together are more or less than five, or anything of this kind in which I perceived a manifest contradiction. (Mediation III)
What all beliefs from rational intuition have in common is the clarity and distinctness of the thought or perception of them. Indeed, in the Third Meditation, Descartes inferred from the clarity and distinctness of the thought of his own existence to the general claim that whatever he perceives very clearly, and very distinctly, is true. His argument was that he could not be mistaken in his belief that he exists when thinking. The only feature of that belief that could signal the infallibility of the belief is its clarity and distinctness. So, he concluded, he could take it that clear and distinct perception is a universal mark of truth.
Nonetheless, when later he is not thinking about such things, doubt can creep back into his mind. This can happen when he re-considers the hypothesis that God has made him so as to be deceived. God could easily, "if he so desired, . . . bring it about that I go wrong even in those matters which I think I see utterly clearly in my mind's eye." This subsequent doubt is, however, "very slight, and so to speak, metaphysical" (Meditation III).
To dispel this slight doubt, Descartes sets about to show that the hypothesis cannot be right. He attempts to prove that God exists and is no deceiver. We shall not examine his "proofs" here, but only note that that they rely on the use of various metaphysical propositions which are clear and distinct to him.
Descartes therefore could not doubt the correctness of his proof at the time he is giving it (so long as he could concentrate on the whole proof at once). Once the proof has been carried out, the slight reason for doubt due to the possible defective character of his faculties has been lifted, and Descartes can be certain about what he is not now perceiving clearly and distinctly. He need only recall the proof, confident that God has given him a memory that does not deceive him.
This intricate strategy was criticized from the outset as threatening to be circular. To dispel his subsequent doubts about what he at one time saw very clearly with his mind's eye, he proves that God exists and is no deceiver. Presumably the proofs themselves depend on his clear and distinct perception of the truth of their premises and their deductive validity. Descartes seems to hold that clear and distinct perception is an infallible standard because God has made him in such a way as not to be deceived in what he clearly and distinctly perceives.
[I]f, wherenever I have to make a judgment, I restrain my will so that it extends to what the intellect clearly and distinctly reveals, and no further, then it is quite impossible for me to go wrong. This is because every clear and distinct perception is undoubtedly something, and hence cannot come from nothing, but must necessarily have God for its author. It author, I say, is God, who is supremely perfect, and who cannot be a deceiver on pain of contradiction; hence the perception is undoubtedly true. (Mediation IV)If Descartes needs to have proved that God exists and is no deceiver in order to know that clear and distinct perceptions are "undoubtedly true," then he is guilty of circularity. He can prove that God exists only by relying on his clear and distinct perception, but he can rely on his clear and distinct perception only by proving that God exists. This problem is known as the "Cartesian Circle."
Descartes himself did not think he was guilty of circular reasoning. Numerous papers have been written on the question of whether Descartes was right, for the reasons he gave or for some other reasons.
Recall that the reason Descartes tried to prove that God exists and is no deceiver was to show how he could dispel doubts that arise when he is not focusing his mental vision clearly on the object of his belief--doubts arising from the possibility that God has made him so as to be deceived. He was not trying to dispel any doubts that he had at the time when he was perceiving clearly and distinctly. As we have seen, Descartes plainly stated that he could not doubt these things at that time. The same holds for the time when he is proving that God exists.
But this does not get Descartes into the clear. The fact that he is absolutely unable to doubt under certain conditions does not seem to imply the truth of what he cannot doubt at that time. He has only shown that his certainty is "subjective," not that it is "objective" (as we might put it now). The problem is how to establish objective certainty at a time before he has proved that God exists.
Descartes seems to have thought he had in fact done so, when setting up clear and distinct perception as the standard for truth. He moved from the impossibility of his being mistaken in his belief that he existed while doubting to the impossibility of his being mistaken in any other matter that he perceived with equal clarity.
This response is still unlikely to satisfy the critics. They would claim that Descartes was not entitled to draw the conclusion that he could not be mistaken without having already established that he was not made by God so as to be deceived in the simplest matters. After all, he admitted that God could be responsible for his being deceived even what he perceives utterly clearly. The objection is that he could not put the genie back into the bottle once he had unleashed it.
David Hume put the objection in terms not of our being created by God with so as to be deceived, but merely of our possibly having defective faculties. He noted that Descartes and other recommend a universal doubtnot only of all our former opinions and principles, but also of our very faculties; of whose veracity, say they, we must assure ourselves, by a chain of reasoning, deduced from some original principle, which cannot possibly be fallacious or deceitful. But neither is there any such original principle, which has a prerogative above others, that are self-evident and convincing: or if there were, could we advance a step beyond it, but by the use of those very faculties, of which we are supposed to be already diffident. (An Enquiry concerning Human Understanding, Section XII, Part I)
The Problem of the External World
One of the most important debates in the epistemology of the modern period is now known as "the problem of the external world." The problem is to find an acceptable validation for claims that we know that there exists, independently of our minds, a world of physical objects in space and time. The ancient Greek skeptics had never sought validation for the general claim that we know that an external world exists. They only questioned our knowledge of the world beyond the way in which it appears.
It was the method of hyperbolic doubt initiated by Descartes which raised the issue of whether we can know that an external physical world exists. Descartes thought that validation is required because of the possibility that we are always dreaming. We may have been made in such a way as to be deceived systematically about whether anything beyond our ideas exists, or we may be victims of a powerful deceiver.
Descartes's strategy in response to these skeptical hypothesis is to show that they could not be true. We can only conceive of their truth when we are ignorant of the source of our existence, but once we have proved that God exists, is our creator, and is no deceiver, we can rule out the possibility altogether.
The argument works like this. We are passive in the reception of some of our ideas (those of "sensory perception"), from which it follows that those ideas have a cause. The cause could be myself, God, or some being less powerful than God, but still capable of producing the ideas which we passively receive. Descartes rules out each of these alternatives.
Descartes gives two reasons that the ideas of sensory perception are not caused by one's self. The first is based on his doctrine that the self is essentially a thinking thing. Roughly, the claim is that the production of an idea is not an act of thinking and therefore myself as thinking thing cannot be its source. The second reason is that "the ideas in question are produced without my cooperation and often even against my will" (Meditation VI). Only the second reason had any influence on subsequent philosophers.
So the source of my "sensory perceptions" must be in some other being, a being suitable to produce them. If such a being were not a body, then we would be deceived about the source, since God has given us no means of detecting such a source, and we have a "great propensity" to believe that sensory perceptions come from physical objects. "I do not see how God could be understood to be anything but a deceiver if the ideas were transmitted from a source other than corporeal things. It follows that corporeal things exist" (Meditation VI).
Skepticism About the External World
This proof is quite elaborate, and it was hardly convincing to philosophers after Descartes. David Hume stated succinctly why Descartes's strategy is suspect. "To have recourse to the veracity of the supreme Being, in order to prove the veracity of our senses, is surely making a very unexpected circuit" (An Enquiry concerning Human Understanding, Section XII, Part I). For an empiricist such as Hume, the requisite proof of the existence of a non-deceptive God as our creator is impossible. The evidence of experience cannot support such a proof, as Hume argued in his Dialogues Concerning Natural Religion.
And if we try to "prove the veracity of our senses" without appealing to God, Hume continued, we run into difficulties, which we will examine shortly. Immanuel Kant, writing in the generation after Hume, was led to conclude that it was a scandal to philosophy and human reason in general that the desired proof had not yet appeared and the existence of external objects was only a matter of faith (Critique of Pure Reason, Preface to the Second Edition). Kant himself tried in various ways to end the scandal.
If we are to give a non-theological proof of the existence of the external world, it would seem to require an appeal to some features of our ideas of sensory perception themselves. One could, as did Descartes, appeal to the feeling that they occur is us without, or even against, our will. And one could appeal as well the coherence of of our sensory perceptions, especially when compared to dreams. The question is whether this is enough to justify our claim to know that sensory perceptions are the products of physical bodies.
Berkeley had answered this question in the negative, and in fact dogmatically denied that physical objects even exist. On his view, our ideas of "sensory perception" are produced by God, which is one of the explanations Descartes had ruled out. Berkeley produced a number of arguments to this effect, but rather than detailing them, we will look at Hume's adaptation of one of them.
Hume recognized (with Descartes) that constancy and coherence are the criteria which we in practice use to distinguish states we take to be induced by external objects from dreams and hallucinations. But, following Berkeley, he asked why we should take coherence to have anything to do with the origin of our perceptual states. Any number of causes could be invoked to account for this coherence, but the mind has no way to get outside (what is now called) the "veil of perception."
It is a question of fact, whether the perceptions of the senses be produced by external objects, resembling them: how shall this question be determined? By experience surely; as all other questions of a like nature. But here experience is, and must be entirely silent. The mind has never anything present to it but the perceptions, and cannot possibly reach any experience of their connection with objects. The supposition of such a connection is, therefore, without any foundation in reasoning. (An Enquiry concerning Human Understating, Section XII, Part I)
One might think that the hypothesis of external objects provides the best explanation of coherence of perceptual experience. But Berkeley argued that a spiritual cause provides a better explanation because the causality of spirits is something of which we have first-hand experience. We can only understand the alleged causality of physical objects in terms of patterns of change in our perceptual states.
It is common at present to blame the modern problem of the external world on a faulty account of perception, which places "ideas" between the human mind and the objects perceived. This was the approach taken by Thomas Reid. So long a what we perceive directly is taken to be internal psychological states of ourselves, we will never be able to "lift the veil" and take a peek at what lies beyond it. We will not discuss this issue any further here, but only note that John Greco has argued recently that the skeptical problem of the external world does not depend on the representational theory of perception (Putting Skeptics in Their Place).
Most contemporary epistemologists think that emphasis on the problem of the external world is misguided, and that it ought to be consigned to the dust-bin of history. Perhaps they are motivated in part by the difficulty in solving it. There are still a few, though, who take the problem seriously and try to deal with it effectively.
We have discussed skeptical objections to the use of rational intuition and perception as standards for regulating assent. Both are supposed to be basic sources of knowledge. We will now discuss the use of probability as a transmissive source of knowledge. The author of the notorious skeptical argument against the use of probability as a standard of evidence is David Hume.
Before going on, we should note that Hume in the Treatise of Human Nature also offered an argument against the use of deduction as a standard (Part I, Book IV, Section I). The basic argument is that to assent rationally, we need assurance that we have carried out the deduction properly, and that the process of checking this leads to an infinite regress.
Let us first take "probability" in the broad sense given it by Locke. A proposition is probable for a person at a time just to that extent that the evidence in favor of it outweighs the evidence against it. One of the things that counts as evidence for the truth of a proposition is the degree to which it conforms with what we already believe. When we assent to a proposition that does conform to our beliefs, we raise the probability of our beliefs in all the members of the conforming set of beliefs. This is what occurs in induction.
The general question Hume raised is why the addition of conforming instances increases probability. Let us illustrate the question with an example. Suppose you are drawing from an urn which contains an unknown number of marbles. The first twenty marbles you have drawn have been black. With each draw, the probability that the next marble drawn will be black seems to increase. Why is this so? To vary the case, suppose that nineteen of the first twenty drawn will be black. If it is highly probable that the next one drawn will be black, why is it highly probable?
The answer would seem to turn on the assumption that the marbles you have drawn are a representative sample of the larger population of marbles in the urn. If you assumed that the marbles are arranged in such a way that there was a cluster of black marbles only at the place from which you had been drawing, you would not take the overwhelming preponderance of black marbles to indicate anything about the larger population. Let us call the assumption you need to make an assumption about uniform distribution.
On what basis is the assumption of uniform distribution made? Hume offered two possibilities: the first that it is made a priori, on the basis of pure reason. The second is that it is made on the basis of observation of the course of the draws. (This division of the possibilities is known in the literature as "Hume's Fork.") He notes that there is no justification a priori for making the assumption, as the uneven distribution is as possible as the even distribution.
So we are left to justify the assumption of uniform distribution on the basis of the observation of the draws that have already been made. The argument would be something like this. Every draw (or almost every draw) has yielded a result just like the previous draw. So, with each conforming draw, we have increasing evidence of the uniformity of the distribution of marbles. Therefore, it is probable that the distribution throughout the urn is uniform.
The problem with this response is that it begs the question. The argument depends on the premise that each conforming draw increases the evidence of the uniformity of the distribution. But we have to assume the uniformity of distribution in order to justify our initial claims to probability. So the crucial premise depends on the principle which it is used to establish. As the Pyrrhonians would put it, this is an instance of the mode of circularity.
Hume himself did not put the problem in terms of an artificial activity such as drawing marbles from an urn. His case was the most comprehensive of all: the interrogation of nature as a whole. Philosophers have alleged that every event which takes place does so in conformity to "natural laws" which admit of no exception. Since an exception is always possible (in the sense that it can be imagined), these "laws" are not known a priori. Therefore, they would have to be known from experience.
Now suppose that we have always observed a definite sequence in nature. For example, when one billiard ball strikes another, the second one moves. We would like to generalize these observations inductively and claim that whenever one billiard ball strikes another the second one moves (given that it is not glued to the table, etc.). The generalization is justified only on the presumption that nature is uniform, i.e., that the patterns found at any place and time are to be found in all places and time.
Now the problem arises. We can justify the claim that nature is uniform only by appeal to observation of the past uniform course of nature. How are we to justify this general claim? It would appear that we could only appeal to our experience of the uniformity of nature, and generalize this claim. But as we have already seen, we can generalize from observed uniformity to unobserved uniformity only by assuming the uniformity of nature.
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. (An Enquiry concerning Human Understanding, Section IV, Part II)Many solutions to this problem have been proposed, but there is no agreement about whether any of them are successful. We will discuss briefly below Kant's attempt at solving "Hume's problem."
With the destructive arguments of David Hume, it looked as if empiricism was doomed to crash on the rocks of skepticism. Thomas Reid and some of his fellow Scots attempted to head off the shipwreck by invoking common sense as a new standard that could justify the use of the standards of perception and induction.
The Pyrrhonian skeptic will demand a justification for the use of common sense as a standard. It cannot, he claims, be used to justify itself, and if it does not justify itself, then some other standard must justify it. But what possible standard could be invoked to justify such a sweeping principle such as that "The natural faculties, by which we distinguish truth from error, are not fallacious" (On the Intellectual Powers of Man, Essay VI, Chapter V)? If we try to justify the use of common sense as a standard, we must use the very natural faculties whose use common sense is said to sanction.
Now perhaps we can appeal to a supernatural faculty which allows us to distinguish truth from error in the case of accepting common sense as a standard. But the use of such a faculty cries out for justification, and it is hard to see how one could be given. It seems that the best we can do is to say that the use of common sense as a standard is permissible even though the standard itself cannot be justified. In other words, we take a dogmatic position with respect to this standard.
Immanuel Kant tried to solve the problems of empiricism without a dogmatic appeal to common sense. He employed the conservative strategy of limiting the content of our claims to justification. If we view experience as a connected system of appearances, then we can have a priori knowledge of the basic structure of that experience. Kant used what we now call "transcendental arguments" to show what must be the case if our experience is to be at all possible.
Kant tried to do what his predecessors had failed to do—to prove that an external world of physical objects exists. The main premise of his "Refutation of Idealism" is that we can use experience to situate our internal states with respect to time. It is then claimed that we can do so only if we can refer these states to an external system of physical bodies. One way of understanding his claim is that the coherence of our "outer" perceptions must represent an objective fact about physical objects if we are to be able to justify beliefs about our internal states which in fact are justified. (Click here for more.)
Regarding probability and induction, Kant took the first of the two paths of Hume's fork. We can establish through a transcendental argument, and not through experience, that nature is uniform. The gist of the argument is that the uniformity of nature is a necessary condition for the ordering of events in time. By taking the path of reason, Kant thought he could establish something even stronger than what Hume denied about probability: that we can discover necessary truths about the course of nature. (Click here for more.)
Kant's exposition of his anti-skeptical position is extremely difficult, and for this reason we will not pursue it here. We can make two comments about his general position. First, Kant accepts a certain kind of skepticism. Our claims to knowledge extend only as far as experience can take us. What Kant called "things in themselves" are unknowable.
Second, all Kant seems to have shown is a conditional: if experience is ordered in a certain way, then it is necessarily that nature be uniform. This is something Hume could grant. But he could at the same time claim that we are never justified in accepting the antecedent of the conditional, that experience is ordered in the way Kant says it is. Thus Kant has either proved less than he set out to prove (that nature is uniform), or else his proof is defective. It either makes an unsupported dogmatic assumption that experience is suitably ordered, or else covertly supports the claim that experience is ordered by the assumption of the uniformity of nature, which would beg the question.
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