Bratty, Lord Denning definition
An act done by the muscles without may control by the mind
Examples:
Hill V Baxter: bees
D must have lost full control over their actions
Broome v Perkins: D driving without care or attention but could still steer and brake which held you must loose full control
If D was at fault - no defence
Bailey - if D knew by only eating sugar he would suffer the condition there is no defence