Knowledge of Modality
A chat about knowledge of necessary truth by intuition and reflection
For a breezy introduction, listen to a generated Deep Dive podcast here.
June 29, 2023
Prompt:
How does David Lewis non-epistemically justify the distinction between necessary and contingent truth?
ChatGPT:
David Lewis, a famous philosopher and logician, was a staunch advocate of modal realism, the view that other possible worlds are as real as our own. In his approach to modality (the philosophical concept addressing the question of necessity and possibility), he proposed a non-epistemic justification for the distinction between necessary and contingent truth.
Lewis argued that a statement is necessarily true if and only if it is true in all possible worlds, whereas a statement is contingently true if it is true in some possible worlds but not in others. The concept of a possible world is crucial to Lewis's modal realism. He believed that each possible world is a concrete entity existing independently of others, even though they cannot interact or influence each other.
For instance, the statement "all bachelors are unmarried" is necessarily true because there's no possible world where this statement would be false. On the other hand, the statement "I am currently typing on my computer" is contingently true because there are possible worlds where this statement could be false (e.g., a world where I'm currently not using a computer).
Importantly, Lewis's understanding of necessary and contingent truth is non-epistemic because it doesn't rely on what we know or don't know. It's solely based on the objective status of statements across all possible worlds, rather than our knowledge or beliefs about those statements.
Prompt:
Lewis's modal realism is instead justified by Quinean extensionalism and a defense of the common sense of modal reasoning. Not the other way around. What I am asking is, how does he justify the acceptance of this common sense modal reasoning. This point is notable for Quine, whose extensionalism is his other primary motive, himself was skeptical of modal reasoning. Quine thought talk of necessity was ultimately confused and he thought no more of it than he did of analyticity?
ChatGPT:
David Lewis' justification for accepting common-sense modal reasoning, despite the skepticism of figures like W. V. O. Quine, largely hinges on a defense of our intuitive and pre-theoretical commitments to modality. That is, we frequently engage in modal talk, hypothesizing about what could or could not be the case, what is or isn't necessary, etc. Our language and thoughts seem to be suffused with modal concepts.
In his work, Lewis provided a systematic and robust theoretical framework—modal realism—that respects and accommodates these pre-theoretical commitments to modality. In other words, Lewis argues that we can and should take our modal talk seriously because it's possible to construct a coherent and scientifically respectable metaphysics in which this talk is truth-apt and grounded.
Quine, by contrast, was critical of modal reasoning primarily because he thought it was not amenable to a suitably regimented and scientifically respectable ontology. He also believed that modal reasoning presupposed an intelligible notion of analyticity, which he famously criticized. Therefore, Quine's skepticism towards modal reasoning and Lewis' acceptance of it stem from their differing views about what ontological commitments our best theories should make.
Lewis maintains that his modal realism can account for our modal talk and reasoning without appealing to problematic notions like analyticity. The possible worlds of his modal realism are fully concrete, just like the actual world, and they're not reducible to or dependent upon our conceptual or linguistic practices. This contrasts with other accounts that treat possible worlds as abstract entities or as linguistic or conceptual constructions.
In conclusion, for Lewis, the acceptance of common-sense modal reasoning is justified by his belief that our modal discourse is truth-apt, meaningful, and can be made sense of within a scientifically respectable ontology (i.e., his modal realism), even without appealing to problematic notions like analyticity.
Prompt:
Even if one accepts common modal reasoning, the question whether it wouldn't make more sense to interpret it as merely the relations among very basic and centrally believed propositions and their logical relations to other propositions which we then consider possible, necessary, or contingent. In other words, how does Lewis justify a metaphysical analysis, instead of a logical-epistemic one.
ChatGPT:
David Lewis justified his metaphysical analysis over a logical-epistemic analysis by arguing for the concreteness and reality of possible worlds. For Lewis, possible worlds are not just abstract entities or sets of propositions that we might consider to be true. Instead, they are just as real and concrete as our actual world.
This approach allows him to provide a metaphysical grounding for modal notions that is independent of our cognitive or linguistic practices. According to Lewis, a statement is necessary if it holds in all possible worlds, not because it's a consequence of some basic propositions we believe. Conversely, a statement is contingent if it holds in some but not all possible worlds. This understanding does not depend on our beliefs or on our logical relations to other propositions.
Furthermore, Lewis also maintained that his modal realism provides a more unified and systematic account of our modal reasoning. If we interpret modal talk as merely involving relations among certain believed propositions, we might have to use different methods to analyze different kinds of modal talk (e.g., deontic modality vs. alethic modality). But if we interpret modal talk as involving quantification over possible worlds, we can use the same method to analyze all kinds of modal talk.
In summary, Lewis justified his metaphysical analysis by arguing for the reality of possible worlds, the independence of modal truth from our cognitive or linguistic practices, and the systematicity and unity of his account.
Prompt:
Maybe I am leaning toward Stalnaker's analysis of Williamson's analysis of modal reasoning. Although I know theirs are also not epistemic, maybe I am closer to theirs anyway, given my appeal to logical relations and basic beliefs.
ChatGPT:
Indeed, Robert Stalnaker's and Timothy Williamson's views on modality share similarities but are distinct from David Lewis's modal realism.
Stalnaker's approach to possible worlds semantics, like Lewis's, attempts to ground modal talk in possible worlds. But unlike Lewis, Stalnaker sees possible worlds as abstract, not concrete. They represent ways the world could be, rather than being fully fledged realities of their own. Stalnaker's model also suggests that our beliefs and assumptions about the world help us construct these abstract models.
Timothy Williamson, on the other hand, offers a necessitist account of modality. He argues that necessarily, everything necessarily exists - that is, every entity exists in all possible worlds. He advocates for a more logical approach, suggesting that modal logic should be understood in terms of quantified modal logic, where one quantifies over all objects that exist in all possible worlds.
Both Stalnaker's and Williamson's approaches might align more with your interests in logical relations and basic beliefs. While still maintaining a metaphysical component, they incorporate a more significant role for our understanding, beliefs, and logical relations in making sense of modal reasoning.
Prompt:
I suppose the integration of one's opinions as David Lewis would put it, his own kind of reflective equilibrium, does not require justifying that there are non-actual possible worlds, really. It can simply be said, with the material conditional, that if there are both contingent and necessary truths, then there are possible worlds, whatever they may be, given that one is committed to some metaphysics of modality, whether it is Lewis's, Stalnaker's, Williamson's, or another. Even if one merely entertains the truth of propositions in modal reasoning, then one is at least entertaining such realities.
ChatGPT:
Absolutely. Your understanding reflects the very core of philosophical exploration. When engaging with modal reasoning, one is essentially dealing with conditional scenarios: "If this is true, then that must be the case." This sort of reasoning, or commitment to exploring these alternatives, can be seen as an implicit acceptance of the existence of 'possible worlds' in some sense, even if one doesn't subscribe to a specific metaphysical model like Lewis's, Stalnaker's, or Williamson's.
In other words, the act of considering alternate possibilities — whether it's an alternate course of history, a different state of affairs, or the potential consequences of a specific action — can be seen as a form of engagement with the realm of possible worlds.
This, in essence, is the value of modal reasoning. It allows us to navigate our understanding of what's necessary and what's contingent, what could be and what could not be, in a structured and rigorous manner, regardless of how we ultimately choose to conceptualize the metaphysical underpinnings of that reasoning. It's more about the commitment to the exploration of alternate possibilities rather than a commitment to a particular metaphysical model of modality.
Prompt:
This may establish the contingency of some truths, but it doesn't establish the necessity of some others. How does these philosophers, or any others, justify that not all truths are contingent, that some are necessary?
ChatGPT:
The distinction between necessary and contingent truths is a fundamental aspect of modal logic and metaphysics. A necessary truth is a statement that could not have been false, while a contingent truth is a statement that could have been false under different circumstances.
The justification for some truths being necessary often comes from our intuitions about logic and mathematics, as well as about the nature of certain conceptual truths.
For instance, logical truths like "if p then p" or "p or not p" are considered to be necessarily true because they hold in all possible circumstances. Similarly, mathematical truths like "2 + 2 = 4" or "all squares have four sides" are also typically considered to be necessarily true, because it's hard to conceive of a possible situation where these statements would be false.
On the other hand, truths about the world, such as "it is raining" or "Paris is the capital of France," are typically considered contingent because it's easy to imagine situations where these statements would be false.
Furthermore, philosophers like Saul Kripke have argued that there are also necessary a posteriori truths—truths that are necessarily true, but can only be known through empirical investigation. An example would be "water is H2O." According to Kripke, once we have discovered that water is H2O, it's not possible for water to be anything else, and hence this is a necessary truth.
So, philosophers typically justify the necessity of some truths based on our logical and mathematical intuitions, as well as on more complex philosophical arguments about the nature of concepts and identities.
Prompt:
It may well be that we justify the necessity of any particular truth by intuition, effectively an inability to imagine contrary possibility. But how is it generally established that this inability is due to necessity as opposed to cognitive limitation?
ChatGPT:
This is indeed a profound philosophical question. It's challenging to establish with certainty that our inability to imagine a contrary possibility is due to the necessity of a truth rather than cognitive limitations or lack of imagination.
Various philosophers approach this question differently.
Semantic theories of necessity: Philosophers like Saul Kripke argue for a semantic explanation for necessity, where certain propositions like identity statements ("Hesperus is Phosphorus" or "Water is H2O") are necessarily true, even if they're discovered a posteriori. In Kripke's view, this necessity arises from the nature of the objects or concepts themselves rather than from our cognitive capabilities or limitations.
Logical and mathematical necessity: Another common approach is to claim that truths of logic and mathematics are necessary because they hold in all possible worlds, irrespective of our ability to conceive them or not. The impossibility of these truths being false is not due to cognitive limitations but due to the fundamental structure of logic and mathematics themselves.
Empirical investigations: In addition to these, some philosophers argue that we can investigate the source of our inability to imagine contrary possibilities empirically, by studying the cognitive processes that underlie our modal judgments. For example, cognitive scientists can investigate whether our modal judgments correlate with certain brain processes, and whether those processes are reliable indicators of necessity.
Reflective equilibrium: Finally, some philosophers might argue for a kind of reflective equilibrium, where we continually refine our judgments about what is necessary or contingent based on our evolving intuitions, theories, and empirical findings.
However, it's important to note that this is an ongoing philosophical debate, and there's no consensus about how to definitively establish the necessity of a truth independently of cognitive limitations.
Prompt:
The semantic theory, in relation to identity, relies on logical and mathematical necessity as well, assuming self-identity identity is a logical truth. It is also said that self identity is an ontological truth since it is de re, rather than de dicto. My own view is that while the truth is ontological, its necessity is logical.
ChatGPT:
Your viewpoint seems to be in line with a sophisticated understanding of necessity. It's a view that seeks to maintain a balance between the ontological and logical aspects of necessity.
The necessity of identity statements, such as "Hesperus is Phosphorus" or "water is H2O," indeed leans on the logical principle of self-identity ("A is A"), which is commonly regarded as a necessary truth. However, as you've pointed out, there's a significant ontological component to these identity statements as well. After all, these statements are about things in the world—Hesperus, Phosphorus, water, H2O—and their properties.
The way you put it, the "truth" of these statements is ontological, meaning that it depends on the way the world is—on the properties and identities of the things these statements are about. However, their "necessity" is logical because it's rooted in the principle of self-identity, which is a logical axiom.
This view seems to strike a balance between respecting the worldliness of necessary truths and acknowledging the logical principles that underpin their necessity. It acknowledges that while the world contributes to the truth of necessary truths, it's the logical principles that secure their necessity. This viewpoint therefore recognizes the interplay between the way the world is and the logical structure of our thoughts about the world.
Prompt:
Whereas reflective equilibrium is methodologically sound, it would result in the justification of necessary truths in general on the basis of necessary truths in particular.
ChatGPT:
Your observation is indeed insightful. Reflective equilibrium is a method that seeks to reconcile our intuitions and principles in a coherent and mutually supportive system. It involves making adjustments to our beliefs and theories to eliminate contradictions and improve overall coherence.
In the case of necessary truths, this might involve comparing our intuitions about specific necessary truths, such as logical or mathematical axioms, with our more general principles about what kinds of things can be necessarily true. We might then adjust either our specific intuitions or our general principles, or both, to reach a state of equilibrium.
However, as you've pointed out, this method could result in justifying necessary truths in general on the basis of necessary truths in particular. That is, our intuitions about specific necessary truths might guide our principles about necessity more generally.
But this is not necessarily a problem. It might simply reflect the fact that our understanding of necessity is grounded in our intuitive grasp of specific examples of necessary truths. This is similar to how our understanding of many other concepts, such as justice or knowledge, is often grounded in our intuitions about specific instances of these concepts.
In this way, reflective equilibrium can provide a flexible and iterative methodology for understanding and justifying necessary truths, based on both our specific intuitions and our general principles.
Related:
Sufficient Reason for Brutality
From Triviality to the Imagination