AHMAD ALMAHOZI

Courtesy: GPT

1. Background: Regularity theory of causation

The Humean regularity theory (and its later refinements) treats causation as a matter of constant conjunction or lawlike regularity:

A causes B if and only if events of type A are regularly followed by events of type B.

Mackie (in The Cement of the Universe, 1974) was initially sympathetic to this Humean line, but came to think that it faced fatal counterexamples. One of these was supplied by Hans Reichenbach’s “common cause principle” and the Reichenbach conjunctive fork.

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2. The Reichenbach problem

Reichenbach observed that correlations between two events (say, the ringing of two bells, or the hands on two clocks) can arise not because one causes the other, but because both share a common cause.
• Suppose:
• Event C (a hidden common cause) produces both A and B.
• Then A and B will be regularly conjoined (correlated).

From a regularity standpoint, it looks like A causes B (since whenever A happens, B tends to happen), but in reality both are effects of C.

This is Reichenbach’s problem: the regularity theory cannot distinguish between genuine causal sequences and mere correlations due to common causes.

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3. The “Hand–Reichenbach problem” (sometimes also called the Hand‐Reichenbach counterexample)

The specific illustration Mackie discusses involves the movement of the hands of a clock.
• The hour hand (A) and the minute hand (B) always move in strict regularity with each other.
• On a regularity account, this would make it look as if the hour hand causes the minute hand to move (or vice versa).
• But in fact, neither hand causes the other. Instead, both are effects of a common internal mechanism of the clock (C).

So we have:
• Regularity present: A ↔ B.
• But causal direction absent: neither A → B nor B → A.
• The true causal structure: C → A and C → B.

This is the Hand–Reichenbach problem: the hands’ regular correlation is spurious from the perspective of causation. The regularity theory misclassifies correlation as causation.

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4. Mackie’s reaction

Mackie concluded that regularity is not sufficient for causation.
• He therefore developed his well-known account of causation in terms of INUS conditions:
• A cause is an Insufficient but Non-redundant part of an Unnecessary but Sufficient condition for the effect.
• On this account, the movement of the hour hand is not an INUS condition for the movement of the minute hand — both are rather INUS conditions within larger causal complexes involving the clock mechanism.

In other words, the INUS analysis can correctly rule out the “hand” example as non-causal, whereas pure regularity theory cannot.

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5. Why this matters

The Hand–Reichenbach problem shows why mere regularity or correlation cannot capture causation. It highlights the need to look at causal structures (common causes, mechanisms, counterfactual dependencies) rather than just surface patterns of events.

This line of thought eventually pushed philosophers toward:
• Mackie’s INUS analysis,
• Reichenbach’s own common-cause principle,
• later probabilistic accounts,
• and interventionist/mechanistic theories in contemporary philosophy of science and neuroscience.