Peter Geach

Peter Geach was a British philosopher who taught at the University of Birmingham and the University of Leeds, working in logic, philosophy of language, philosophy of mind and philosophy of religion. In 'Assertion' he argued that moral words keep the same meaning in unasserted contexts such as conditionals, which expressivist theories struggle to explain. This 'embedding' or Frege–Geach problem remains a central objection to non-cognitivism.

Key ideas

• The Frege–Geach problem: 'lying is wrong' means the same when asserted and when embedded in 'if lying is wrong, then...', so its meaning cannot be just an expressed attitude.
• Valid moral arguments such as modus ponens require moral sentences to function logically like descriptive ones.
• 'Good' is an attributive adjective: to be good is always to be a good something.
• Identity is relative to a sortal concept: 'the same' always means 'the same F'.

Major works

• Mental Acts (1957)
• Reference and Generality (1962)
• Assertion (1965)
• The Virtues (1977)

In the conversation

• Debate Moral disputes are disagreements in attitude, not in belief, once the facts are agreed Pieter van Dijkhuizen · 5 replies
• Question Can someone walk me through the Frege–Geach problem without the jargon? Siobhán Keane · 4 replies