In the previous chapter, we explored the hypothetico-deductive method, which many find intuitively appealing and aligned with the common perception of the scientific method. This method primarily serves as a means of confirmation, guiding how observations can support a theory and bolster our confidence in it. However, there are numerous approaches to scientific confirmation, such as naive inductivism and probabilistic models. Despite the diversity of these models, they all face a significant challenge: the problem of induction.
Hume's Argument Against Induction
David Hume famously argued that induction is not rationally justified. If Hume's argument holds, it suggests that we cannot rationally trust any ampliative inference—those inferences where the conclusion extends beyond the information contained in the premises. This would fundamentally undermine any theory of scientific confirmation, given that science inherently involves making ampliative inferences to provide knowledge about the world.
The Nature of Inductive Inference
Inductive inference involves generalizing from observed instances to unobserved instances. For example, from the premise that all observed fires have been hot, we might infer that the next fire we observe will also be hot. This process appears reasonable and can also be framed probabilistically, where we infer that the next fire will probably be hot based on most past fires being hot.
The Justification Challenge
Hume's core question is: Why should we believe that inductive inferences are justified? There are two principal methods for justifying a rule of inference:
1. Deductive Justification: We could try to show that the rule guarantees the truth of the conclusion if the premises are true. However, induction cannot be justified deductively because it is ampliative; the fact that all observed fires have been hot does not logically entail that the next fire will be hot.
2. Inductive Justification: We might argue that because induction has worked in the past, it will continue to work in the future. However, this justification is circular and question-begging—it assumes the reliability of induction to prove the reliability of induction.
Hume argues that since induction cannot be justified by either method, it is fundamentally irrational. Inductive arguments provide no reason to believe their conclusions, equating inductive reasoning to mere guesswork.
Counter-Inductivism
To further illustrate his point, Hume invites us to consider a counter-inductivist, someone who uses a rule of inference that draws opposite conclusions from inductive reasoning. For instance, a counter-inductivist might infer from the fact that all observed fires have been hot that the next fire will not be hot. If we challenge the counter-inductivist by pointing out the practical success of induction, they can respond by applying their own counter-inductive rule, claiming that since induction has been successful in the past, it will not be successful in the future. This demonstrates that using past success to justify a method is inherently circular.
The Uniformity of Nature
One might attempt to resolve the problem by appealing to the principle of the uniformity of nature, which posits that the future will resemble the past. If we assume this principle, induction seems rational. However, this principle itself requires justification, and it can only be justified inductively, leading to the same circular reasoning.
Moreover, even if we accept a weaker version of this principle—that nature is uniform to some extent—this does not guarantee that the regularities we observe are genuine rather than spurious. For instance, apparent regularities, such as a certain precinct always voting for the winning presidential candidate, may just be coincidental.
Responses to Hume's Problem
Several responses to Hume's problem have been proposed:
1. Dismissal: Some argue that the problem is irrelevant because induction clearly works, as evidenced by the success of science. However, this does not address the philosophical concern about the rational foundation of induction.
2. Conceptual Argument: Peter Strawson suggests that questioning the rationality of induction is incoherent because our concept of rationality inherently includes inductive reasoning. However, this approach can justify any method of inference, including absurd ones like the Azande chicken oracle, suggesting it may be too relativistic.
3. Pragmatic Justification: Hans Reichenbach offers a pragmatic argument, suggesting that even if we cannot justify induction theoretically, it is still our best bet. If nature is uniform, induction will be successful; if not, no method will succeed. While this argument may justify the practical use of induction, it does not guarantee its reliability.
Hume's problem of induction remains a profound challenge to the rational foundation of science. While various responses offer ways to cope with the problem, none provide a definitive solution. Induction's lack of rational justification suggests that the foundations of scientific knowledge might be more tenuous than we would like to believe. This problem highlights the ongoing philosophical task of critically examining the methods by which we claim to know the world.