When is the abettor liable although the person abetted is not?

A completed crime is the last step of a sequence the criminal law dissects into four stages — and the dissection is not academic: liability attaches differently at each stage, and the boundaries define the oldest protections of criminal law (mere intention and preparation are, as a rule, lawful). Stage 1 — Intention (mens only): the formation of the criminal purpose in the mind. The rule: not punishable — cogitationis poenam nemo patitur (no one suffers punishment for mere thought). Law punishes conduct, not contemplation; the privacy of the mind is liberty's outer wall. The exceptions appear only where the intention is manifested into punishable expression (conspiracy's agreement; incitement's communication) — the uncommunicated intention, however wicked, is outside criminal law. Stage 2 — Preparation: acts designed to facilitate the intended crime — buying the poison, casing the bank, sharpening the khukuri, arranging the getaway. General rule: not punishable — preparation does not yet harm, and everyday conduct (buying rope) is too ambiguous to criminalize. The exceptions (memorize the list): where the preparation itself is dangerous or its proximity convincing, statutes criminalize it directly — possession-type offences (weapons, counterfeit materials, drugs in quantity); preparations against State security (the treason/ espionage class where the State cannot wait); organized preparation for armed robbery in some jurisdictions; the modern expansion of membership/possession offences. Nepal's…