1. A method for retroactively analyzing a log of context attribute values associated with a user, each context attribute value contained by the log is stored with an indication of a corresponding data item and an indexing value, the method comprising: receiving from at least one application a specification for analyzing values among the stored values, the specification specifying one or more context attributes, a range of indexing values that are effective times associated with data items, and an analysis technique applicable to the data items, the analysis technique includes at least one of summarization, combination, new value generation, ongoing analysis, pattern analysis, or retrospective application of rules, the retrospective application of rules includes a determination of a result that would have been produced had a rule been applied to analyze the specified context attribute values at the time the values were generated, the rule configured for analyzing context attribute values in real-time to produce a result and is adoptable for future real-time application when it is determined that a successful result would have been produced had the rule been applied to analyze the specified context attribute values at the time the values were generated; retrieving stored values for the specified context attributes associated with the user of a computing device within the specified range of indexing values, a first subset of the stored values for one of the specified context attributes being associated with a first source device, and a second subset for the stored values for the one of the specified context attributes being associated with a second source device; applying the specified analysis technique to the retrieved values to produce an analysis of the context attributes that infers information representing at least one of the user's physical activity, emotional state, or cognitive load; and operating the at least one application to receive and use the inferred information.