Online research in philosophy

This article argues that: (i) Defeasible reasoning is the use of distinctive procedures for belief revision when new evidence or new authoritative judgment is interpolated into a system of beliefs about an application domain. (ii) These procedures can be explicated and implemented using standard higher-order logic combined with epistemic assumptions about the system of beliefs. The procedures mentioned in (i) depend on the explication in (ii), which is largely described in terms of a Prolog program, EVID, which implements a system for interactive, defeasible reasoning when combined with an application knowledge base. It is shown that defeasible reasoning depends on a meta-level Closed World Assumption applied to the relationship between supporting evidence and a defeasible conclusion based on this evidence. Thesis (i) is then further defended by showing that the EVID explication of defeasible reasoning has sufficient representational power to cover a wide variety of practical applications of defeasible reasoning, especially in the context of decision making.

My starting point is some widely accepted and intuitive ideas about justified, well-founded belief. By drawing on John Pollock’s work, I sketch a formal framework for making these ideas precise. Central to this framework is the notion of an inference graph. An inference graph represents everything that is relevant about a subject for determining which of her beliefs are justified, such as what the subject believes based on what. The strengths of the nodes of the graph represent the degrees of justification of the corresponding beliefs. There are two ways in which degrees of justification can be computed within this framework. I argue that there is not any way of doing the calculations in a broadly probabilistic manner. The only alternative looks to be a thoroughly non-probabilistic way of thinking wedded to the thought that justification is closed under competent deduction. However, I argue that such a view is unable to capture the intuitive notion of justification, for it leads to an uncomfortable dilemma: either a widespread scepticism about justification, or drawing epistemically spurious distinctions between different types of lotteries. This should worry anyone interested in well-founded belief.

One of the most striking characteristics of human beings is their ability to function successfully in complex environments about which they know very little. In light of our pervasive ignorance, we cannot get around in the world just reasoning deductively from our prior beliefs together with new perceptual input. As our conclusions are not guaranteed to be true, we must countenance the possibility that new information will lead us to change our minds, withdrawing previously adopted beliefs. In this sense, our reasoning is “defeasible”. The question arises how defeasible reasoning works, or ought to work. In particular we need rules governing what a cognizer ought to believe given a set of interacting arguments some of which defeat others. That is what is called a “semantics” for defeasible reasoning, and this chapter will propose a new semantics that avoids certain clear counter-examples to all existing semantics.

Joshua Thurow offers a defence of the claim that if a belief is defeasible by non-experiential evidence then it is defeasible by experiential evidence. He responds to an objection which I make against this claim, and offers two arguments in support of his own position. I show that Thurow's response misconstrues my objection, and that his supporting arguments fall short of their goal.

The problem of induction and the problem of Cartesian/brain-in-the-vat skepticism have much in common. Both are instances of a general problem of defeasible justification . I use the term "defeasible justification" to refer to a relation between a piece of evidence.

In a recent and interesting paper “Experientially Defeasible A Priori Justification,” Joshua Thurow argues that many a priori justified beliefs are defeasible by experience. The argument takes the form of an objection against Albert Casullo’s recent book, A Priori Justification, where Casullo, according to Thurow, denies that if a justified belief is non-experientially defeasible, then that belief is also experientially defeasible. This paper critically examines Thurow’s two arguments in the first two sections I–II. In the last section, III, an alternative line of argument for Thurow’s thesis is suggested that employs other parts of the framework that Casullo provides—especially the thesis of overdetermination of justification. It will be argued that the prospects for this suggestion are brighter than for bothof Thurow’s arguments.

This paper gives an explication of our intuitive notion of strength of justification in a controversial debate. It defines a thesis' degree of justification within the bipolar argumentation framework of the theory of dialectical structures as the ratio of coherently adoptable positions according to which that thesis is true over all coherently adoptable positions. Broadening this definition, the notion of conditional degree of justification, i.e.\ degree of partial entailment, is introduced. Thus defined degrees of justification correspond to our pre-theoretic intuitions in the sense that supporting and defending a thesis t increases, whereas attacking it decreases, t's degree of justification. Moreover, it is shown that (conditional) degrees of justification are (conditional) probabilities. Eventually, the paper explains that it is rational to believe theses with a high degree of justification insofar as this strengthens the robustness of one's position.

An argument is self-defeating when it contains defeaters for some of its own defeasible lines. It is shown that the obvious rules for defeat among arguments do not handle self-defeating arguments correctly. It turns out that they constitute a pervasive phenomenon that threatens to cripple defeasible reasoning, leading to almost all defeasible reasoning being defeated by unexpected interactions with self-defeating arguments. This leads to some important changes in the general theory of defeasible reasoning.

Deductive and inductive logic confront this skeptical challenge: we can justify any logical principle only by means of an argument but we can acquire justification by means of an argument only if we are already justified in believing some logical principle. We could solve this problem if probative arguments do not require justified belief in their corresponding conditionals. For if not, then inferential justification would not require justified belief in any logical principle. So even arguments whose corresponding conditionals are epistemically dependent upon their conclusions--epistemically self-supporting arguments--need not be viciously circular. R. B. Braithwaite and James Van Cleve use internalist and externalist versions of this strategy in their proposed solutions to the problem of induction. Unfortunately, their arguments for self-support are unsound and any theory of inferential justification that does not require justified belief in the corresponding conditionals of justification-affording arguments is unacceptably arbitrary. So self-supporting arguments cannot be justification-creating.