For a contract to be legally enforceable in English law, three conditions must be met: offer, acceptance, consideration and an intention to create legal relations. In Fisher, it was established that a shop keeper who displays a flick knife for sale in a window along with a price cannot be considered to have made an offer.
Instead, the display was an ‘invitation to treat’. The offer would only occur once a customer offered to buy the knife.
The law was subsequently amended by the Restriction of Offensive Weapons Act 1961 (9 & 10 Eliz 2 c. 22) so that anyone who ‘exposes or has in his possession for the purpose of sale or hire’ a flick knife has committed an offence. The nature of what constitutes an offer and an invitation to treat in contract law remains the same, however.