2.1. Normal disagreements

To begin, please note that, for clarity, I assume a realist view about knowledge, beliefs, objects, facts, and properties (this does not mean that we cannot work out the same analysis by rejecting realism or assuming other metaphysical constraints, but I will not explore them here). An individual mentally represents what is happening in the world and therefore has true or false beliefs, depending on the state of affairs.

The parties have different beliefs when they are engaged in a normal disagreement. For example, Jennifer believes that P is true and James believes that P is false. Both beliefs are about the same fact, object, or property. While Jennifer believes that Mount Everest is more than eight thousand meters high, James believes that Mount Everest is not more than eight thousand meters high (this reference to the same subject matter, Mount Everest, ensures that the dispute is not merely verbal). Please note that a disagreement does not require Jennifer to believe that P and James that ¬P, or vice versa. James can exhibit disagreement through a third doxastic attitude: doubting or suspending judgment. James does not reject that P but suspends his judgment or doubts that Mount Everest is more than eight thousand meters high is true. It is uncontroversial that agreement requires that both parties believe either that P or that ¬P at the same time. Therefore, if both parties consider P and only one of them believes either that P or that ¬P and the other party does not hold the same belief, then they are disagreeing.

The arguments I develop in the following sections depend on this reading of disagreement. Of course, this strong interpretation of disagreement can be challenged. In that case, I think that this interpretation is still a reasonable interpretation of disagreement and should be seriously considered.

Coming back to the subject, normal disagreements are normal because they are resolvable. The parties share some or many of the same beliefs, “preferences” (in the case of moral arguments), and evaluative procedures in such a way that, through rational argumentation (arguments and logical evaluation of the evidence), they can resolve it. This kind of disagreement “takes place within a context of widely shared beliefs and preferences” (Fogelin Reference Fogelin1985: 6).

Although both parties initially disagree, dialogue and rational argument expose their shared beliefs. Physicians, to give an example, soon realize that they share beliefs about fundamental laws of logic, the testimony of the senses, the operation of instruments, and the proper procedures for delivering a good diagnosis. During the discussion, one or both parties begin to see their methodological errors or the employment of the instruments and begin to acquire the relevant information and avoid errors in reasoning and in the application of various methods and instruments. The difference in evidence is never so big as to generate deep disagreements. Both parties share their procedures, or some evidence, for determining when a belief is justified and true.

2.2. Deep disagreements

A deep disagreement is like a normal one in that Jennifer believes that P is true and James believes that P is false – or the parties hold different doxastic attitudes about P – and both beliefs are about the same fact, object, or property (the disagreement is genuine). However, they differ in the rest: the parties do not share background beliefs and preferences or adequate epistemic standards for assessing whether the other party’s evidence E or F justifies the belief that P or ¬P – or suspending judgment on P.

It seems that dialogue and argumentative activity cannot resolve disagreement because disagreement does not involve superficial beliefs but the deepest ones. Both parties dispute over superficial beliefs as well, but this stems from a deeper dispute: Jennifer’s disagreement over what Fogelin refers to as “fundamental principles” or “framework propositions” (1985: 8). “What I have called deep disagreements are generated by conflicts between framework propositions,” says Fogelin (8). These propositions underlie the entire theoretical structure of a belief system, and the conflict between them is the fundamental characteristic of deep disagreements independently of whether we take them to be fundamental epistemic principles (Lynch Reference Lynch2012; Smith and Lynch Reference Smith and Lynch2021) or hinges (Prichard Reference Prichard2021; Ranalli Reference Ranalli2018b), for example. The important point is that “they provide the framework or the structure within which reasons can be marshaled, where marshaling reasons is typically a matter of citing facts that others already know or of arranging facts in a way that their significance becomes clear” (Fogelin Reference Fogelin1985: 5).

Another fundamental characteristic of deep disagreements is that the basic propositions are “fundamental structural propositions” that support “a whole system of propositions that support each other” (9) and, for this reason, if the parties disagree about the fundamental structural propositions, they will disagree on all the others. Given this holistic and hierarchical interrelationship between propositions, the difference in the fundamental structural propositions determines which facts and the methods of evaluation will be considered.Footnote ¹ Appealing to more facts, removing biases and prejudices, clarifying terms, and removing inconsistencies do not seem to be capable of resolving the disagreement.

We can think of the debate over employing or not animals in experimentation as a plausible candidate for deep disagreement. One party argues that it is immoral to use animals for testing new drugs. The other party considers it moral because it helps to save humans and even to save animal lives themselves. Those who do not accept animal experimentation call attention to the cruelty of the experimental methods like deprivation or forced feeding, exposure to infectious diseases, wounds and burns, and death by suffocation (Humane Society International n.d.). They point to false positives and negatives in experimental treatments and limited predictions due to the physiological differences between humans and animals (Hackam and Redelmeier Reference Hackam and Redelmeier2006; Horrobin Reference Horrobin2003). The other part points out that animal experimentation has been useful in the treatment of cancer, multiple sclerosis (Joyce et al. Reference Joyce, Fratta, Fisher and Acevedo-Arozena2011), and tuberculosis in humans (Van Rhijn et al. Reference Van Rhijn, Godfroid, Michel and Rutten2008), as well as feline leukemia (Dunham Reference Dunham2006) and leprosy in armadillos (Charles Reference Charles2003). They also maintain that it is essential to use them because the anatomical and physiological similarity between animals and humans is higher than other alternatives (we share 99% of our genes with chimpanzees and 98% with mice). Alternative systems like using human cells or computational models do not provide the same experimental conditions as those of living organisms to study, for example, high blood pressure or blindness.

The resolution of this disagreement requires sharing some fundamental structural beliefs relevant to the discussion, but these seem to be absent from the debate.Footnote ² The fundamental structural belief of those who advocate animal experimentation is that saving lives and promoting the health of both humans and animals is worth the pain caused by animal experimentation. Instead, the fundamental structural belief of those who oppose it is that saving lives and promoting the health of both humans and animals is no justification for inflicting pain on animals in the pursuit of experimentation.

2.3. Radical skepticism

Skepticism consists of the doxastic attitude of doubt or rejection about the existence of knowledge, and throughout history, various kinds of skepticism have existed. Two of the most outstanding skeptical versions are Pyrrhonism and Cartesianism. In this paper, I do not offer a detailed analysis of each one. I only mention four general characteristics coming from Cartesianism and Pyrrhonism that we can use to characterize radical skepticism and thus be able to show its connection with deep disagreements. Greek skepticism, in its Pyrrhonic version, resorts to “modes” of suspending judgment and derives its contemporary importance from a subset of these: “the modes of Agrippa” (Comesaña and Klein Reference Comesaña, Klein and Zalta2019). Agrippa’s trilemma reveals three characteristics or deficiencies of justification: dogmatism, infinite regress, and circularity. In all three cases, justification is deficient and unable to join with an individual’s true belief to create knowledge.

Cartesian skepticism reveals another fundamental deficiency of justification: It is fallible because, given certain evidence, it is always possible to imagine a scenario capable of questioning or rejecting any proposition that assigns knowledge to an individual.Footnote ³ The Cartesian skeptical argument is usually expressed as follows:

Therefore,

P can be any other ordinary proposition and ¬Q any other radical skeptical scenario (and P and Q must be incompatible). The Cartesian skeptic makes plain that fallible justification is not sufficient to believe that the skeptical scenarios are false. Only infallible justification is sufficient to reject skepticism. The justification needs to guarantee truth for the Cartesian skeptic.

Thus, justification is a necessary component of knowledge and radical skepticism reveals four fundamental deficiencies of justification that can preclude knowledge.

As we will see below, there are local kinds of radical skepticism (which are not global) that have the same four characteristics mentioned above. Radical skepticism about animal morality does not doubt or reject all claims to knowledge, but only those concerning moral values. We will see that while Jennifer says, “I know that it is not moral to experiment on animals,” James says, “you do not know that it is not moral to experiment on animals.” Jennifer asserts P, and James asserts ¬P. James relies, explicitly or implicitly, on some or all of the above principles to doubt Jennifer’s claim. Jennifer’s justification is dogmatic, circular, infinite, or fallible.

As we can see, certain parallelism regarding the intractability of discussions between skepticism and deep disagreements is emerging. I will address these similarities in the next section.

3.1. Radical skeptical disagreement is a kind of deep disagreement

Aikin (Reference Aikin2018b, Reference Aikin2018c, Reference Aikin2020) suggests that deep disagreements are a special case of the problem of the criterion. The problem of the criterion consists of consistently answering the following questions: What do we know? And how do we decide in each case whether or not we have knowledge? Answering the first question means having already answered the second and, therefore, having a criterion to decide what we know. Likewise, answering the second question means having already answered the first and, therefore, knowing which cases are knowledge. It seems that we cannot break the circle.

Aikin finds in the problem of the criterion the evidential difficulties posed by Agrippa’s trilemma: dogmatism, circularity, and infinite regress. Knowing that we know (first question), we presuppose the solution to the trilemma. That is, having a non-dogmatic, circular, or an infinite regress criterion. Knowing how we know (second question) also presupposes the solution to the trilemma: having the criteria. For these reasons, “no belief is justified, and by extension, no disputes are rationally resolved” (2018b: 1020). These same properties, he argues, characterize deep disagreement, and therefore “the phenomenon of deep disagreement is best seen as an instance […] of the problem of the criterion –insofar as there is persistent controversy, argument cannot resolve what is at issue” (2018b: 1021). However, the characteristics of deep disagreements and skepticism mentioned above suggest that skeptical disagreement is an instance of deep disagreement.Footnote ⁴ As Melchior notes, skeptical arguments “can be reinterpreted as arguments about the preconditions and limits of persuasive argumentation, explaining why we cannot persuade skeptical characters. Moreover, these reinterpretations reveal certain instances of deep disagreement” (2021: 21).

Before showing why radical skeptical disagreement is a case of deep disagreement, let us look at the characteristics that differentiate them. A deep disagreement consists of two individuals or groups of individuals disputing over proposition P. Jennifer, for example, asserts P and James asserts ¬P. An argument with a radical skeptic has the same general structure. Jennifer holds that P and James holds that ¬P. The fundamental difference lies in the kind of propositions. The discussion with the radical skeptic is only about sentences that assert or reject knowledge (the disagreement is metaepistemic in the sense that what is at issue is the epistemic state of having or lacking knowledge), but deep disagreements can deal with other topics, such as the morality of animal experimentation or the actual existence of infinity.

Two kinds of doxastic attitudes usually appear in deep disagreements: assertion and rejection. In contrast, in a disagreement with a radical skeptic, the skeptical doxastic attitude is usually one of doubt or suspension of judgment, but it can also be of rejection of knowledge (James, the skeptic, rejects Jennifer’s putative knowledge that she is not a brain in a vat. But note that this rejection does not commit the skeptic to the belief that he is a brain in a vat). For ease of exposition, I focus and run the argument based on the doxastic attitude of doubt because it is the most usual skeptical doxastic attitude (at least at the base level. See below): The parties disagree because one believes that P and the other suspends judgment about P (remember that for the parties to agree, they need to believe either P or ¬P).Footnote ⁵ However, bear in mind that skeptical party can also reject P.

Another difference between deep disagreements and radical skepticism occurs when the latter is global. Although, as I show below, deep disagreements share essential features with global radical skeptical disagreement, the former cannot be charged with being an inconsistent position. Global radical skepticism denies the existence of any kind of knowledge, and for this reason, one common objection is that it is an inconsistent position. Even skeptical versions whose doxastic attitude is the suspension of judgment, such as Pyrrhonism, can be charged with having inconsistent beliefs.Footnote ⁶ This does not happen in deep disagreements where a global radical skeptic does not participate because the parties only reject knowledge from the other party about the subject under discussion.

Despite these differences, radical skepticism and deep disagreements share a similar logical structure. Skeptical disagreement is a kind of deep disagreement at the metalevel – that is, at the level of epistemic evaluation, where one asserts, rejects, or doubts that someone else possesses knowledge.

Finally, let me point out that I refer to specific individuals, James and Jennifer, only for illustrative purposes. Nothing hangs on the psychology of particular individuals. What matters is that people cannot reach agreement, even if they are rational and all the evidence is on the table.

Let us return to the disagreements between Jennifer and James. Jennifer claims, “I know that I am not a brain in a vat” and James denies this. Jennifer and James disagree at the metalevel. This is a genuine disagreement that exhibits the four skeptical characteristics mentioned above. Jennifer claims, “I know that I am not a brain in a vat” on the basis that she can see her hands and then James quickly points out that Jennifer does not have a justified true belief by pointing out the inadequacy of her justification. Her evidence is deficient, the advocate of Agrippa’s trilemma argues, because if she does not justify it with further evidence, then she is dogmatic. If she justifies it with additional evidence E1, then she needs further evidence E2 for justifying E1, and so on, but she cannot infinitely justify a proposition, so her claim is ultimately unjustified. She can justify the empirical justification of her belief (she sees her hands) by employing more empirical evidence (she sees that she sees her hands), but she would be employing circular justification. Finally, there is no guarantee, the Cartesian skeptic argues, that Jennifer’s justification is sufficient because there may always be a skeptical scenario in which she is not justified.Footnote ⁷

Disputes with a skeptic fit the general characteristics identified by Fogelin in his seminal paper. The parties do not share sufficient beliefs and procedures to reach agreement. Moreover, the parties differ in their fundamental structural principles. The radical skeptic defends the fundamental structural principle that a person possesses knowledge only if the belief in it is error-free, non-dogmatic, non-circular, or does not require infinite justifications. On the other hand, it seems ridiculous to the non-skeptic to demand such a condition on a person’s beliefs. For a non-skeptic, the fundamental structural principle is that it is enough to have good fallible reasons to believe that P. These fundamental principles determine the course of the dispute. The parties will not be able to reach an agreement because, although they can understand non-fundamental beliefs and procedures such as “Mount Everest is a mountain,” “I believe that P and you reject that P,” and “engaging in rational argument is a good strategy for resolving debates,” their fundamental structural beliefs determine how much justification the parties demand for this kind of propositions. For the skeptic, they do not constitute cases of knowledge because there is no justification or it is not enough. For the non-skeptic, there is justification and it is enough.Footnote ⁸

3.2. Deep disagreements and skepticism covary

Another essential feature of the close relationship between radical skepticism and deep disagreements is their covariance. Since skeptical disagreement is a special case of the latter, their close relationship is not surprising.

Let us consider the following sentences:

1. (1) “I am not a brain in a vat.”

2. (2) “I doubt that you are not a brain in a vat.”

Jennifer asserts (1) and James, the radical skeptic, asserts (2).

Jennifer asserts (1) because she holds the fundamental structural principle that having sense experiences is sufficient to assert (1). On the other hand, James asserts (2) mainly because he holds the fundamental structural principle that having sense experiences is not sufficient to assert (1).

This deep disagreement is not about knowledge but about beliefs about the world. But it certainly covaries with radical skepticism at the metalevel. Sentences (1) and (2) are connected to sentences (1*) and (2*).

1. (1*) “I know that I am not a brain in a vat.”

2. (2*) “You do not know that you are not a brain in a vat.”

(1*) and (2*) are also deep disagreements, but they are about propositions that assert or reject knowledge. James believes (2*) because it is based on some or all the four fundamental skeptical principles (James thinks, for example, that (1*) would be true only if Jennifer had infallible justification). Jennifer instead believes (1*) on the basis of her fundamental structural belief that it is enough to have fallible evidence to believe (1*).

Please note that holding a skeptic fundamental structural principle at the metalevel implies holding a fundamental structural principle at the object level – and its derived beliefs – and vice versa. James believes (2) on the basis of an object-level fundamental principle, but he may also believe (2) and hold this object-level fundamental principle on the basis of the conjunction of the object-level fundamental structural principle and the metalevel four skeptical principles or on the basis of one or all the four skeptical principles alone. In the same way, James believes (2*) on the basis of a metalevel fundamental skeptical principle or principles, but he may also believe (2*) and hold one or all fundamental skeptical principles on the basis of the object-level fundamental structural principle that having sense experiences is not sufficient to know that one has a body. Note also that, given that the fundamental structural principles at the object level and at the metalevel imply each other, one cannot resolve an object-level deep disagreement unless we resolve the parallel skeptical disagreement and vice versa.

Now, I just gave a skeptical example, the classic brain in a vat thought experiment, but covariance also occurs in other cases of deep disagreement, like the dispute over animal testing. One party defends (a) and the other defends (b).

1. (a) “It is immoral to experiment on animals.”

2. (b) “It is not immoral to experiment on animals.”

Jennifer believes (a) and James believes (b). This is a deep disagreement because Jennifer’s fundamental structural belief is that it is immoral to cause animal suffering when the life or health of humans or other sentient beings is in danger. James, in contrast, has the fundamental structural belief that it is moral to inflict pain on animals when the life or health of humans or other sentient beings is in danger. The parties cannot resolve the debate because the basic structural principles are different. No matter how many facts and logical arguments they bring to the table, it seems that the result will always be an open dispute. The parties may agree on all the facts and logical arguments about animal testing, like the amount of pain produced, the benefits and drawbacks of experimentation, or the ethical theories like utilitarianism and deontology, but they will not reach an agreement because these do not seem to settle the matter.

In these cases of deep disagreement at the base level, there is deep skeptical disagreement at the metalevel about the following sentences:

1. (a*) “I know that it is immoral to experiment on animals.”

2. (b*) “You do not know that it is immoral to experiment on animals.”

Jennifer believes (a*) and James believes (b*). Deep disagreement at the object level ((a) and (b)) leads to radical skeptical disagreement at the metalevel ((a*) and (b*)) and vice versa. Although (a*) and (b*) do not seem to be cases of radical skepticism, they qualify as such insofar as they meet some or all the four skeptical characteristics. James denies (a*) because Jennifer does not meet his fundamental structural belief of having infallible, non-dogmatic, non-circular justification or needs an infinite chain of justifications. On the other hand, Jennifer’s fundamental structural principle is that she does not need to meet one or all of James’ skeptical requirements.

3.3. Responses to some objections

Of course, one may object that there is no skeptical disagreement because James is not really a skeptic but just a believer in the morality of animal experimentation: James denies (a*) on the grounds that Jennifer’s belief is just false; James does not care about whether Jennifer has justification or not. On the other hand, a skeptic rejects (a*) because Jennifer is not justified. In other words, both the skeptic and the believer in the morality of animal experimentation reject Jennifer’s knowledge that (a), but they diverge because, while the skeptic thinks that justification is the element of knowledge that is missing, the believer in the morality of animal experimentation thinks that the missing element is truth. Unlike the skeptic, James rejects that Jennifer knows that (a) because her beliefs lack truth. The overall situation is that there is no skeptical disagreement. However, appearances are misleading. Let us examine this situation closely.

In order to reject (a) and (a*) without being a skeptic, James needs to assume the following principle:

In James and Jennifer’s case, James would be a dogmatic who thinks that he can know that ¬(a) and ¬ (a*) even if 1) he lacks justification for believing ¬(a) and ¬(a*) and 2) Jennifer has justification or is more justified than James in believing (a) and (a*).Footnote ⁹

However, this proposal is not successful. There is a skeptical disagreement after all. Let us see why. For a start, if both James and Jennifer assume (c*), there is still a skeptical disagreement between them because Jennifer has to resort to skeptical arguments as the only rational way for her to disagree with James. Jennifer cannot be a dogmatic herself without falling into a contradiction. In this scenario, based on (c*), James rejects (a) and (a*) despite his lack of justification, and by this very principle, Jennifer rejects ¬(a) and ¬(a*).Footnote ¹⁰ But now Jennifer needs to accept, by (c*), that James is right and she is wrong, but she cannot do so because, also by (c*), James is wrong and she is right. James finds himself in the same situation. The only way James or Jennifer can reject each other’s beliefs is by being skeptical about those beliefs and thus rejecting the dogmatic principle (c*).

One option is that either James or Jennifer rejects (c*). If James assumes (c*) and Jennifer rejects it, then Jennifer disagrees with James because she is playing the skeptic role and rejects James’ beliefs on the grounds that they are dogmatic, which is part of the skeptical fundamental structural belief that rejects dogmatic, circular, infinite, and fallible justification. If Jennifer assumes (c*) and James rejects it, then James disagrees with Jennifer because he is playing the skeptic role. A skeptical disagreement is, after all, taking place at the metalevel.

The other option is that James and Jennifer reject (c*). In this case, both are skeptics. Both reject each other’s beliefs on the grounds that they are not supported by non-dogmatic, non-circular, finite, and infallible justification. A skeptical disagreement is also taking place at the metalevel after all. No inconsistency arises here, unlike the situation in which both accept (c*).

In sum, in all cases, even if one of the parties is dogmatic, they engage in a skeptical disagreement at the metalevel; one of the parties of a deep disagreement is a radical skeptic at the metalevel.

One may still argue that James can dogmatically hold ¬(a) and ¬(a*), while Jennifer can hold (a) and (a*), respectively, as long as they do not become aware of the intrinsic inconsistency of being dogmatic and thus assuming (c*). The problem with this response is that James and Jennifer will eventually know that they hold inconsistent beliefs, and if this awareness does not take place, it is because the disagreement is not yet deep in Fogelin’s sense of a clash of framework propositions (the deepest kind), where the parties have clarified the terms and recognized all the facts and arguments.

I know the reader may hold more reservations about the existence of a skeptical disagreement at the metalevel because James or Jennifer’s rejection of each other views does not seem to be any kind of skepticism. Well, the logical structure is the same and the differences are accidental. As I mentioned earlier, skepticism and other varieties of deep disagreement can vary in the number of advocates: Skeptics tend to be part of a minority of people and the parties of a deep disagreement tend to be equal in number, but the logical structure of the skeptical and deep disagreement is the same in both cases. Even though it seems odd to say that Jennifer is a skeptic about (b) and (b*), she is really a skeptic in a world where most people hold that (b) and (b*). I am not presenting a discussion that is sensitive to superficial features, like the number of people on one or the other side of the debate. I am interested, as Fogelin stated, in the logic of disagreements and the underlying logical structure implies that both Jennifer and James are skeptics.

Still, someone might retort that skeptics and non-skeptics still argue and sometimes reach consensus. Of course, I acknowledge that epistemologists can and indeed sometimes reach agreement, but this hardly can take place between the deeply disagreeing parts of the dispute. The deep disagreement about whether, for instance, one knows that one is not a brain in a vat remains unresolvable to this day: Skepticism about the external world is still a live issue. Now, as Fogelin pointed out, some deep disagreements, such as the discussion about animal experimentation, seem to be rooted in “preferences” and thus escape rational resolution (it does not mean that there are other ways in which they can be resolved). Moreover, I do not contend that the parties who deeply disagree do not or cannot agree on many non-deep issues. I am just arguing that they hardly could resolve the issue about the deepest principles.

Now, why not simply say that deep disagreements are just a kind of peer disagreements, a discussion between two equally knowledgeable epistemologists in this case? I think that deep disagreements are quite different from peer disagreements because the first is necessarily deep: They arise even when both parties have properly processed the evidence and no one has misapplied a rule. Peer disagreements need not be deep in Fogelin’s sense of “a clash of fundamental principles” that resist further rational argumentation and evidence. One continuously has disagreements between experts in biology, physics, medicine, etc., but they tend to eventually reach a resolution as new evidence and reasons accumulate. These cases aren’t cases of deep disagreement.

In conclusion, skeptical disagreement is a kind of deep disagreement that covaries with a deep disagreement at the object level. Animal experimentation is just an example. There may be many other similar cases. We can point out several in philosophy: the dispute between realists and antirealists, causality versus teleology, and the debate on the ontology of numbers and moral values (Arroyo Reference Arroyo, Arroyo, Matienzo, Marafioti and Santibáñez Yañez2014: 17–18). These disagreements run so deep that it is common to compare them to disputes between skeptics and non-skeptics.

3.3. Absolute depth

We have seen that deep disagreements and skepticism covary and both exhibit the characteristics enunciated by both Fogelin and the skeptics. Moreover, given covariation, resolving a deep disagreement requires resolving the logical problem present in the very structure of radical skepticism. The depth exceeds any characteristic of everyday discussions. The evidential demands outweigh cases as thorny as discussions about climate change, the ethics of self-driving cars, and even the possibility of counting the number of stars in the universe. In these cases, it is possible to access new evidence, and there is no logical reason to think that they are not resolvable, at least in an ideal context. But the problem with deep disagreements is that they are absolutely deep because an infinite amount of evidence, different kinds of evidence (expert testimony, internal or external, etc.), does not seem to settle the matter. The problem of deep disagreement may continue indefinitely. Jennifer will continue to believe (1) and (a) and James will continue to believe (2) and (b) because they hold competing fundamental structural beliefs about sense perception and animal ethics.

If their discussion moves from the object level to the level of epistemic evaluation, Jennifer will continue to hold (1*) and (a*) and James will continue to hold (2*) and (b*) because the latter espouses one or all skeptical fundamental structural principles: fallibility, dogmatism, circularity, or appealing to an infinite chain of justifications.Footnote ¹¹ The parties can understand the terms, the evidence, and the arguments adduced and appeal to a greater quantity and quality of evidence, but the debate may be indeed insoluble due to its absolute depth.

If Fogelin is correct, then “there are disagreements, sometimes on important issues, which by their nature, are not subject to rational solution” (1985: 11). Does this mean that there is no known solution to deep disagreements? Is there any way to solve them? In this paper, I have suggested that deep disagreements are not rationally irresoluble, but it leaves open the possibility that some of them can be resolvable.

Perhaps not all deep disagreements are necessarily as absolutely deep as radical skepticism seems to be. There are probably versions of less radical disagreements where the parties would reach an agreement if they get more information and methods. Several philosophers hold that deep disagreements come in degrees (Aikin Reference Aikin2018a, Reference Aikin2020; Henderson Reference Henderson2020; Lavorerio Reference Lavorerio2020). Even Fogelin seems to adopt this perspective when he asks and answers, “What happens to arguments when the context is neither normal nor nearly normal? The answer that seems forced upon us is this: to the extent that the argumentative context becomes less normal, argument, to that extent, becomes impossible” (1985: 7). This is a promising line of investigation worthy of consideration for understanding and perhaps resolving deep disagreements.