Christian List

Standard impossibility theorems on judgment aggregation over logically connected propositions either use a controversial systematicity condition or apply only to agendas of propositions with rich logical connections. Are there any serious impossibilities without these restrictions? We prove an impossibility theorem without requiring systematicity that applies to most standard agendas: Every judgment aggregation function (with rational inputs and outputs) satisfying a condition called unbiasedness is dictatorial (or effectively dictatorial if we remove one of the agenda conditions). Our agenda conditions are tight. When applied illustratively to (strict) preference aggregation represented in our model, the result implies that every unbiased social welfare function with universal domain is effectively dictatorial.

While a large social-choice-theoretic literature discusses the aggregation of individual judgments into collective ones, there is much less formal work on the transformation of judgments in group communication. I develop a model of judgment transformation and prove a baseline impossibility theorem: Any judgment transformation function satisfying some initially plausible conditions is the identity function, under which no opinion change occurs. I identify escape routes from this impossibility and argue that the kind of group communication envisaged by deliberative democrats must be 'holistic': It must focus on webs of connected propositions, not on one proposition at a time, which echoes the Duhem-Quine 'holism thesis' on scientific theory testing. My approach provides a map of the logical space in which different possible group communication processes are located.

1 August 2003 Karl Popper noted that, when social scientists are members of the society they study, they may affect that society. If the individuals to whom a theory initially applies come to understand that theory, then this understanding may affect their behaviour in such a way that the theory ceases to be applicable. This may be called the problem of reflexivity. In this paper, we identify such a problem in an apparently unlikely area: in the area of Condorcet’s famous jury theorem. Suppose that each individual member of some decisionmaking body has a greater than 0.5 chance of making a correct judgment, and suppose further that all individuals’ judgments are independent from each other. Then the jury theorem states that the majority will make a correct judgment with a probability approaching 1 as the number of individuals increases. We argue that, if the individuals come to understand the jury theorem, then they may cease to make independent judgments, thereby undermining one of the conditions for the application of the theorem. Specifically, we suggest that the individuals may be faced with a temptation to free-ride on the epistemic efforts of others. We first develop the problem in some detail and then ask whether there are any escape routes that can protect the jury theorem against the effect of reflexivity.

Suppose a committee, expert panel, or other group is making judgments on some issues, where these may be not just yes/no-questions, such as whether a defendant is guilty, but also variables with many possible values, such as macroeconomic or meteorological variables or travel directions. Furthermore, there may be interconnections between different issues, as in the case of economic or climate variables. How can the group arrive at “intelligent” collective judgments, based on the group members’ individual judgments? We investigate three challenges raised by this judgment-aggregation problem. First, reasonable methods of aggregation (such as defining the collective judgment for each issue as the average or median judgment) can produce inconsistent collective judgments. Secondly, many methods of aggregation are manipulable by strategic voting. Finally, not all methods of aggregation are conducive to tracking the truth on the issues in question. We prove new impossibility or possibility theorems on all three challenges, identifying what it takes to produce collective judgments in a consistent, non-manipulable, and truth-tracking manner and thereby to achieve collective intelligence through aggregation. Overall, the median method, though imperfect, performs reasonably well. We also note the relevance of our analysis for non-human group decisions. (The manuscript has been accepted for publication in Philosophical Transactions of the Royal Society B and is to appear in 2026.)

This chapter offers a critical assessment of the ‘exclusion argument’ against free will, which may be summarized by the slogan: ‘My brain made me do it, therefore I couldn’t have been free.’ While the exclusion argument has received much attention in debates about mental causation (‘could my mental states ever cause my actions?’), it is seldom discussed in relation to free will. However, the argument informally underlies many neuroscientific discussions of free will, especially the claim that advances in neuroscience seriously challenge our belief in free will. The chapter introduces two distinct versions of the argument, discusses several unsuccessful responses to it, and then presents the authors’ preferred response. This involves showing that a key premise—the ‘exclusion principle’—is false under what the authors take to be the most natural account of causation in the context of agency: the difference-making account. The chapter finally revisits the debate about neuroscience and free will.

What action-guiding judgments should we rely on in cases of moral uncertainty, when we divide our credence among competing moral views and assign credences between 0 and 1 to propositions such as “action x is better than action y”? We show that the problem of “moral uncertainty resolution” can be viewed as a “belief binarization” problem: how to arrive at all-out (“accept/reject”) judgments on some propositions based on our credences in them. Looking at moral uncertainty through this lens yields a diagnostically useful impossibility result, which identifies a tradeoff between four initially plausible conditions on a moral uncertainty resolution rule.

In this paper, I argue that no theory of consciousness can simultaneously respect four initially plausible metaphysical claims—namely ‘first-person realism’, ‘non-solipsism’, ‘non-fragmentation’, and ‘one world’—but that any three of the four claims are mutually consistent. So, theories of consciousness face a ‘quadrilemma’. Since it will be hard to achieve a consensus on which of the four claims to retain and which to give up, we arrive at a landscape of competing theories, all of which have pros and cons. I will briefly indicate which kinds of theories correspond to the four horns of the quadrilemma.

Standard impossibility theorems on judgment aggregation over logically connected propositions either use a controversial systematicity condition or apply only to agendas of propositions with rich logical connections. Are there any serious impossibilities without these restrictions? We prove an impossibility theorem without requiring systematicity that applies to most standard agendas: Every judgment aggregation function (with rational inputs and outputs) satisfying a condition called unbiasedness is dictatorial (or effectively dictatorial if we remove one of the agenda conditions). Our agenda conditions are tight. When applied illustratively to (strict) preference aggregation represented in our model, the result implies that every unbiased social welfare function with universal domain is effectively dictatorial.

How can several individuals’ probability assignments to some events be aggregated into a collective probability assignment? Classic results on this problem assume that the set of relevant events—the agenda—is a σ-algebra and is thus closed under disjunction (union) and conjunction (intersection). We drop this demanding assumption and explore probabilistic opinion pooling on general agendas. One might be interested in the probability of rain and that of an interest-rate increase, but not in the probability of rain or an interest-rate increase. We characterize linear pooling and neutral pooling for general agendas, with classic results as special cases for agendas that are σ-algebras. As an illustrative application, we also consider probabilistic preference aggregation. Finally, we unify our results with existing results on binary judgment aggregation and Arrovian preference aggregation. We show that the same kinds of axioms (independence and consensus preservation) have radically different implications for different aggregation problems: linearity for probability aggregation and dictatorship for binary judgment or preference aggregation.

Can groups be rational agents over and above their individual members? We argue that group agents are distinguished by their capacity to mimic the way in which individual agents act and that this capacity must “supervene” on the group members' contributions. But what is the nature of this supervenience relation? Focusing on group judgments, we argue that, for a group to be rational, its judgment on a particular proposition cannot generally be a function of the members' individual judgments on that proposition. Rather, it must be a function of their individual sets of judgments across many propositions. So knowing what the group members individually think about some proposition does not generally tell us how the group collectively adjudicates that proposition: the supervenience relation must be “setwise,” not “proposition‐wise.” Our account preserves the individualistic view that group agency is nothing mysterious but also suggests that a group agent may hold judgments that are not directly continuous with its members' corresponding individual judgments.

One of the hallmark themes of Karl Popper’s approach to the social sciences was the insistence that when social scientists are members of the society they study, then they are liable to affect that society. In particular, they are liable to affect it in such a way that the claims they make lose their validity. “The interaction between the scientist’s pronouncements and social life almost invariably creates situations in which we have not only to consider the truth of such pronouncements, but also their actual influence on future developments. The social scientist may be striving to find the truth; but, at the same time, he must always be exerting a definite influence upon society. The very fact that his pronouncements do exert an influence destroys their objectivity.” (Popper 1963.

In solving judgment aggregation problems, groups often face constraints. Many decision problems can be modelled in terms the acceptance or rejection of certain propositions in a language, and constraints as propositions that the decisions should be consistent with. For example, court judgments in breach-of-contract cases should be consistent with the constraint that action and obligation are necessary and sufficient for liability; judgments on how to rank several options in an order of preference with the constraint of transitivity; and judgments on budget items with budgetary constraints. Often more or less demanding constraints on decisions are imaginable. For instance, in preference ranking problems, the transitivity constraint is often contrasted with the weaker acyclicity constraint. In this paper, we make constraints explicit in judgment aggregation by relativizing the rationality conditions of consistency and deductive closure to a constraint set, whose variation yields more or less strong notions of rationality. We review several general results on judgment aggregation in light of such constraints.

While there has been much discussion of whether AI systems could function as moral agents or acquire sentience, there has been very little discussion of whether AI systems could have free will. I sketch a framework for thinking about this question, inspired by Daniel Dennett’s work. I argue that, to determine whether an AI system has free will, we should not look for some mysterious property, expect its underlying algorithms to be indeterministic, or ask whether the system is unpredictable. Rather, we should simply ask whether we have good explanatory reasons to view the system as an intentional agent, with the capacity for choice between alternative possibilities and control over the resulting actions. If the answer is “yes”, then the system counts as having free will in a pragmatic and diagnostically useful sense.

Group decisions must often obey exogenous constraints. While in a preference aggregation problem constraints are modelled by restricting the set of feasible alternatives, this paper discusses the modelling of constraints when aggregating individual yes/no judgments on interconnected propositions. For example, court judgments in breach-of-contract cases should respect the constraint that action and obligation are necessary and sufficient for liability, and judgments on budget items should respect budgetary constraints. In this paper, we make constraints in judgment aggregation explicit by relativizing the rationality conditions of consistency and deductive closure to a constraint set, whose variation yields more or less strong notions of rationality. This approach of modelling constraints explicitly contrasts with that of building constraints as axioms into the logic, which turns compliance with constraints into a matter of logical consistency and thereby conflates requirements of ordinary logical consistency and requirements dictated by the environment . We present some general impossibility results on constrained judgment aggregation; they are immediate corollaries of known results on judgment aggregation.

Political theory, sometimes also called “normative political theory”, is a subfield of the disciplines of philosophy and political science that addresses conceptual, normative, and evaluative questions concerning politics and society, broadly construed. Examples are: When is a society just? What does it mean for its members to be free? When is one distribution of goods socially preferable to another? What makes a political authority legitimate? How should we trade off different values, such as liberty, prosperity, and security, against one another? What do we owe, not just to our fellow citizens, but to people in the world at large? In this article, we review the methodology of a core branch of contemporary political theory: the one commonly described as “analytic” political theory. In Section 1, we briefly demarcate the scope of political theory. In Section 2, we comment on the analysis of political concepts. In Section 3, we introduce the notions of principles and theories, as distinct from concepts. In Section 4, we discuss the methods of assessing such principles and theories, for the purpose of justifying or criticizing them. In Section 5, we review a recent debate on how abstract and idealized political theory should be. In Section 6, finally, we discuss the significance of disagreement in political theory. Although we cover established ground, we do so from an angle that will be somewhat unfamiliar to at least some political theorists – namely an angle inspired by the philosophy of science. We have chosen this angle with a view to systematizing the activity of analytic political theorizing so as to make its connections with other fields of philosophy and positive science more transparent.

This paper provides an introductory review of the theory of judgment aggregation. It introduces the paradoxes of majority voting that originally motivated the field, explains several key results on the impossibility of propositionwise judgment aggregation, presents a pedagogical proof of one of those results, discusses escape routes from the impossibility and relates judgment aggregation to some other salient aggregation problems, such as preference aggregation, abstract aggregation and probability aggregation. The present illustrative rather than exhaustive review is intended to give readers new to the field of judgment aggregation a sense of this rapidly growing research area.

In the theory of judgment aggregation, it is known for which agendas of propositions it is possible to aggregate individual judgments into collective ones in accordance with the Arrow-inspired requirements of universal domain, collective rationality, unanimity preservation, nondictatorship and propositionwise independence. But it is only partially known for which agendas it is possible to respect additional requirements, notably non-oligarchy, anonymity, no individual veto power, or implication preservation. We fully characterize the agendas for which there are such possibilities, thereby answering the most salient open questions about propositionwise judgment aggregation. Our results build on earlier results by Nehring and Puppe (2002), Nehring (2006) and Dietrich and List (2007a).

In judgment aggregation, unlike preference aggregation, not much is known about domain restrictions that guarantee consistent majority outcomes. We introduce several conditions on individual judgments su¢ - cient for consistent majority judgments. Some are based on global orders of propositions or individuals, others on local orders, still others not on orders at all. Some generalize classic social-choice-theoretic domain conditions, others have no counterpart. Our most general condition gen- eralizes Sen’s triplewise value-restriction, itself the most general classic condition. We also prove a new characterization theorem: for a large class of domains, if there exists any aggregation function satisfying some democratic conditions, then majority voting is the unique such function. Taken together, our results provide new support for the robustness of majority rule.

All existing impossibility theorems on judgment aggregation require individ- ual and collective judgment sets to be consistent and complete (in some recent results with completeness relaxed to deductive closure), arguably a demand- ing rationality requirement. They do not carry over to aggregation functions mapping pro…les of (merely) consistent individual judgment sets to (merely) consistent collective ones. We prove that, whenever the agenda of propositions under consideration exhibits mild interconnections, any such aggregation func- tion that is "neutral" between the acceptance and rejection of each proposition is dictatorial. We relate this theorem to the literature.

This paper argues that decision theory presupposes free will. Although decision theorists seldom acknowledge this, the way decision theory represents, explains, or rationalizes choice behaviour acquires its intended interpretation only under the assumption that decision-makers are agents capable of making free choices between alternative possibilities. Without that assumption, both normative and descriptive decision theory, including the revealed-preference paradigm, would have to be reinterpreted in implausible ways. The hypothesis that decision-makers have free will is therefore explanatorily indispensable for decision theory. If we regard explanatory indispensability as an indicator of reality in science, decision theorists should embrace the idea of free will.