1. A portable user device including an image or sound sensor, a processor, and a memory, the memory containing software instructions causing the device to perform a method that includes: applying a first classification procedure to information received by said image or sound sensor, to identify a type of said received image or sound information from among plural possible types; applying a second classification procedure to received second information, to identify a scenario, from among plural possible scenarios, the identified scenario comprising the confluence of at least three circumstances, the received second information being different than said received image or sound information; and activating one or more of plural recognition modules based on outputs from the first and second classification procedures, and not activating one or more other of said plural recognition modules; wherein the identified scenario comprises the confluence of at least three circumstances selected from the list consisting of: information from a first camera of said portable user device, information from a second camera of said portable user device, social networking information, and information about a person in proximity to the user.