Logic: Reasoning, Inference and Fallacies
Logic examines correct reasoning, inference and fallacies. The exam tests validity, syllogistic form, Nyaya inference and the naming of fallacies.
Core rules and concepts
- Deduction aims at validity: if premises are true, the conclusion must be true. Soundness requires validity plus true premises.
- Induction generalises from observed cases; it is strong or weak, not valid in the deductive sense.
- Categorical syllogism has major, minor and middle terms; fallacies include undistributed middle and illicit process.
- Nyaya anumana has five members: pratijna (thesis), hetu (reason), udaharana (example), upanaya (application), nigamana (conclusion). It depends on vyapti.
- Hetvabhasa (fallacies of inference): savyabhichara (deviating), viruddha (contradictory), satpratipaksha (counterbalanced), asiddha (unproved), badhita (contradicted by perception).
Worked example
'All wet streets are slippery; this street is wet; therefore it is slippery.' Valid form: All M are P; S is M; therefore S is P. The reasoning is deductively valid. If 'wet' means only slightly damp, the major premise is false, so the argument is not sound.
Nyaya form: 'The hill has fire (pratijna), because it has smoke (hetu), like a kitchen (udaharana), this hill has smoke (upanaya), therefore fire (nigamana).'
Traps
Do not say a valid argument is true. Do not confuse affirming the consequent with modus ponens. Do not call a false premise a fallacy of form. Do not list hetvabhasa without explaining the defect.