Why is the accessory after the fact guilty of a separate offence?

Every crime is an event with a cast: who held the knife, who drove, who planned, who supplied, who sheltered. The law of parties assigns criminal liability across the cast. The analytical question: does criminal law punish only the hand that acts, or also the minds and hands that contributed? Every system answers broadly — liability extends — but by different techniques:• The common law's historical device:principal in the first degree (the actor), principal in thesecond degree (the aider-abettor present at the crime: "standing by, ready to help"), accessorybefore the fact (assistance/encouragement in advance), accessory after the fact (harbouring with knowledge) — a taxonomy built on physical position and timing;• The Subcontinental-Nepali technique: the general-part categories of principal offenderand abettor (the Muluki Ain tradition, continued by the 2074 Code), with abetment's own internal structure (instigation, aid, conspiracy — Unit 5) and separate offences for harbouring- type conduct;• The civil-law device: the unitary "author/participant" (Täterschaft/Teilnahme) with perpetration and participation as grades;• The American modern approach: complicity statutes treating aiders as principals, plus the Pinkerton conspiracy doctrine.