Debate: Is it rational to trust the past?

Induction cannot be justified

Hume was right. There is no non-circular rational justification of induction. We rely on it from habit.

Best argument

Every proposed justification either assumes the uniformity of nature, the principle that the future resembles the past, which is itself an inductive conclusion, or appeals to the past success of induction, which is circular. No one has found a way out in nearly three centuries.

Strongest objection

If this is true, there is no rational difference between expecting the sun to rise and expecting it not to. That seems absurd, and suggests that the standard of 'justification' being demanded is too high.

Defended by

David Hume

Science does not need induction

Science proceeds by conjecture and refutation, not induction. Theories are never confirmed, only tested and possibly falsified.

Best argument

Karl Popper argued that while no number of white swans proves 'all swans are white', a single black swan disproves it. Falsification uses only deduction. Science advances by proposing bold theories and trying hard to refute them. Those that survive are not proven, merely not yet refuted.

Strongest objection

We still have to decide which surviving theory to rely on when building a bridge or taking a medicine, and choosing the best-tested one seems to involve exactly the inductive confidence Popper wanted to avoid.

Defended by

Karl Popper

Induction is rational by definition

Asking whether induction is reasonable is like asking whether the law is legal. Being reasonable about the unobserved simply means reasoning inductively.

Best argument

P. F. Strawson argued that induction is part of what we mean by 'rational belief' about the unobserved. There is no higher standard against which induction could be judged and found wanting. Demanding a deductive justification for induction is a category mistake.

Strongest objection

This seems to dissolve the problem by definition rather than solve it. We can still coherently ask whether a method of reasoning that we call 'rational' actually tends to lead to the truth.

Defended by

P. F. Strawson

Probability can help

Bayesian probability theory explains how evidence should change our confidence, and shows that inductive reasoning is coherent.

Best argument

Bayesians model belief as degrees of confidence updated by evidence according to the laws of probability. Over time, agents who update this way converge on the truth in a wide range of cases. This does not prove the future will resemble the past, but it shows how to reason well given that you start with some reasonable expectations.

Strongest objection

Everything depends on the starting probabilities, the priors. Someone who starts out expecting nature to be irregular will draw very different conclusions from the same evidence. Bayesianism tells you how to update, but not where to begin.

Defended by

Thomas Bayes, Frank Ramsey, Bruno de Finetti

For the full background, variations and open questions, read the entry: Why trust that the future will be like the past?.