Combining Philosophers

Ideas for Peter Geach, Gottfried Leibniz and Aristotle

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2 ideas

5. Theory of Logic / B. Logical Consequence / 1. Logical Consequence
Something holds universally when it is proved of an arbitrary and primitive case [Aristotle]
     Full Idea: Something holds universally when it is proved of an arbitrary and primitive case.
     From: Aristotle (Posterior Analytics [c.327 BCE], 73b33)
     A reaction: A key idea in mathematical logic, but it always puzzles me. If you snatch a random person in London, and they are extremely tall, does that prove that people of London are extremely tall? How do we know the arbitrary is representative?
5. Theory of Logic / B. Logical Consequence / 3. Deductive Consequence |-
Deduction is when we suppose one thing, and another necessarily follows [Aristotle]
     Full Idea: A deduction is a discourse in which, certain things having been supposed, something different from the things supposed results of necessity because these things are so.
     From: Aristotle (Prior Analytics [c.328 BCE], 24b18)
     A reaction: Notice that it is modal ('suppose', rather than 'know'), that necessity is involved, which is presumably metaphysical necessity, and that there are assumptions about what would be true, and not just what follows from what.