4.1 Infancy (Immaturity)
The principle: children below the age of criminal responsibility cannot commit crimes — doliincapax: the law's presumption of incapacity to do wrong. Nepal's Code scheme (by principle): a graded ladder — below the minimum age (10 years under the Code's scheme), no criminal liability at all; then ascending bands of partial capacity with mandatory mitigation (reduced punishment), juvenile-justice procedure under the Children's Act applying to minors (diversion, reformative dispositions, borstal-type custody — the child's welfare principle). Comparative: the doli incapax rebuttable presumption (10–14) of common law's retreat; the ICCPR/Beijing Rules' child-justice standards (rehabilitation over retribution). The analytical points: infancy negates capacity (a status defence, no inquiry into the particular child's understanding below the fixed line); the age at the time of the act governs; the juvenile system's dilemma — protection vs. public alarm at grave juvenile crime.
4.2 Insanity
The principle: no punishment where, at the time of the act, disease of the mind deprived the actor of the capacity to know the act's nature, or its wrongness — furiosus nisi furore solum interfici potest — the madman is punished by his madness alone. The M'Naghten Rules (1843 — the common law's charter, still the exam's core): to establish the defence, it must be proved that at the act's time, under a defect of reason from disease of the mind, the accused (i) did not know the nature and quality of the act, or (ii) did not know it was wrong (legally wrong — the variant tests: morally wrong in the Windfield debate; the US's variants adding irresistible impulse and, in the Model Penal Code, substantial capacity appreciation/control tests). The refinement stack: M'Naghten (cognition) → irresistible impulse (volition) → substantial capacity (MPC) → the abolitionist movement (replacing the defence with mental-health disposals). Nepali law: the Code adopts the M'Naghten-type test (unsoundness of mind negating knowledge of the act or its illegality at the time of the offence) — with the procedural features: burden on the defence (rebutting the sanity presumption), the medical/legal insanity distinction (a mental disorder's diagnosis is not the defence — many disordered offenders are legally sane), the verdict's consequence (acquittal but custodial psychiatric disposal in the public interest — the "not guilty by reason of insanity" and its civil-liberty critiques).
4.3 Mistake
The dichotomy that organizes everything (Unit 3's twin returns):
1. Mistake of fact: a genuine error about the world (the buffalo was, I reasonably believed, mine; the man in the dark was the intruder I thought) — can negative mens rea: if I truly believed the property mine, I lacked the intent to take another's property. Two versions: any honest mistake (subjective — negating intention) vs. reasonable mistake (the negligence- standard requirement for offences of recklessness/negligence, and for some justifications' reasonable-belief conditions);
2. Mistake of law: "everyone is presumed to know the law" — generally no defence (ignorantia juris non excusat), with the qualifications studied in Unit 3 (foreign law, official misleading, unlawfulfulness-knowledge elements). Mistake's defence-function: chiefly mens-rea negation, not a standalone excuse — the accused is acquitted because the prosecution cannot prove the required mental element, not because the law forgives error.
4.4 Intoxication
The rule's architecture:
1. Voluntary intoxication is generally no defence — the policy: one who disables his own judgment cannot profit by it (the DPP v. Majewski rule: a man takes intoxication into the bargain);
2. The exception — intoxication negating specific intent: where the offence requires purpose/intention (murder's intent, theft's dishonest intention), evidence that drunkenness made the actor incapable of forming that intent can reduce murder to manslaughter-type homicide (the intent offence collapses; the recklessness/negligence offence below it remains);
3. Involuntary intoxication (spiked drinks, coerced drugging, unexpected medical reaction) — a full excuse on the insanity-analogy (the actor did not assume the incapacity);
4. Pathological intoxication and drunkenness-with-delirium — treated under insanity's framework (disease of the mind). Nepal's Code: intoxication provisions following the voluntary/involuntary distinction with mitigation only where capacity was substantially destroyed without the actor's fault (describe by principle). The Nepali practice point: intoxicated violence offences — courts distinguishing "deadly intent drowned" (liability for the reckless grade) from "intent never formed."