Abstract: An array processor system is provided with a system to implement a refraction algorithm to prevent incorrect expert system rule firing based on stale or future data, in those production system expert systems which employ content addressable memories for storage of the expert system's facts and its processing control information. The computer system is especially suitable for system which have expert system resources, and there are generic applications of refraction which can be used in any architecture, from scalar to massively parallel, and an associative memory or content addressable memory. The system need not use the RETE algorithm. The computer expert system, has an inference engine and a refraction check mechanism. It is provided with a time stamping mechanism. The computer memory will have working memory elements associated with the processing elements of the array processor. The array processor has a content addressable memory.