A Recursive Semantics for Defeasible Reasoning
A Recursive Semantics for Defeasible Reasoning
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可废止推理的递归语义
DOI：
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发表时间：
2009
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影响因子：
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通讯作者：
J. Pollock
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作者：
J. Pollock
人类最显著的特征之一是，他们能够在他们知之甚少的复杂环境中成功地发挥作用。鉴于我们无处不在的无知，我们不能仅仅根据我们以前的信念和新的知觉输入来演绎推理，在这个世界上生活。由于我们的结论不能保证是真实的，我们必须支持这样一种可能性，即新的信息将导致我们改变主意，收回以前接受的信念。从这个意义上说，我们的推理是“不可行的”。问题出现了，可废止推理如何起作用，或者应该起作用。特别是，我们需要制定规则，规定认知者应该相信什么，因为有一系列相互作用的论点，其中一些论点击败了另一些论点。这就是所谓的可废止推理的“语义学”，本章将提出一种新的语义学，它避免了对所有现有语义的某些明确的反例。1.面对无处不在的无知进行推理人类最显著的特征之一是，他们能够在自己知之甚少的复杂环境中成功地发挥作用。回想一下，你对构成这个世界特征的所有个人事实的了解是多么的少。除了模糊的概括，你还知道中国树上的苹果，一粒粒沙子，甚至辛辛那提的居民？但这并不妨碍你在拜访中国、躺在夏威夷的海滩上或在辛辛那提演讲时吃苹果。我们对个别事实的无知比我们所知的大了许多数量级。当我们转向对一般事实的了解时，情况并没有改善。现代科学给了我们一些概括，我们的经验教会了我们许多更高层次但不那么精确的一般性真理，但我们肯定对大多数一般性真理一无所知。鉴于我们无处不在的无知，我们不能仅仅根据我们以前的信念和新的知觉输入来演绎推理，在这个世界上生活。当我们观察我们实际使用的各种推理时，这一点是显而易见的。我们倾向于相信感知，假设事物是我们看起来的样子，即使我们知道有时不是这样的。我们倾向于认为，我们感知到的事实将保持真实，至少在一段时间内，当我们不再感知它们时，当然，它们可能不会。而且，重要的是，我们将我们的个人观察归纳地结合在一起，形成了关于统计和无例外概括的信念。所有这些推理都不是演绎上有效的。另一方面，我们不能因为基于这种非确凿证据而得出结论而受到批评，因为没有可行的替代方案。我们的非演绎推理使我们的结论是合理的，但不能保证其真实性。由于我们的结论不能保证是真实的，我们必须支持这样一种可能性，即新的信息将导致我们改变主意，收回以前接受的信念。从这个意义上说，我们的推理是“不可行的”。也就是说，它使我们形成信念是合理的，但它可能会被使保持以前合理的信念变得不合理的考虑所“击败”。如果我们要理解理性认知是如何起作用的，我们就必须知道不可行推理是如何起作用的，或者说应该起作用。本章试图回答这个问题。
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. 1. Reasoning in the Face of Pervasive Ignorance One of the most striking characteristics of human beings is their ability to function successfully in complex environments about which they know very little. Reflect on how little you really know about all the individual matters of fact that characterize the world. What, other than vague generalizations, do you know about the apples on the trees of China, individual grains of sand, or even the residents of Cincinnati? But that does not prevent you from eating an apple while visiting China, lying on the beach in Hawaii, or giving a lecture in Cincinnati. Our ignorance of individual matters of fact is many orders of magnitude greater than our knowledge. And the situation does not improve when we turn to knowledge of general facts. Modern science apprises us of some generalizations, and our experience teaches us numerous higher-level although less precise general truths, but surely we are ignorant of most general truths. 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. This is obvious when we look at the varieties of reasoning we actually employ. We tend to trust perception, assuming that things are the way they appear to us, even though we know that sometimes they are not. And we tend to assume that facts we have learned perceptually will remain true, at least for awhile, when we are no longer perceiving them, but of course, they might not. And, importantly, we combine our individual observations inductively to form beliefs about both statistical and exceptionless generalizations. None of this reasoning is deductively valid. On the other hand, we cannot be criticized for drawing conclusions on the basis of such non-conclusive evidence, because there is no feasible alternative. Our non-deductive reasoning makes our conclusions reasonable, but does not guarantee their truth. 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”. That is, it makes it reasonable for us to form beliefs, but it can be “defeated” by considerations that make it unreasonable to maintain the previously reasonable beliefs. If we are to understand how rational cognition works, we must know how defeasible reasoning works, or ought to work. This chapter attempts to answer that question.