1. A controller for a plant comprising a fixed-weight recurrent neural network having at least one external input signal representative of a desired condition of the plant, an output connected as a control signal to the plant, a set of nodes with fixed weight interconnections between said nodes and at least one feedback input interconnecting an output from at least one of said nodes to an input of at least one node, said nodes collectively determining the value of an output of the fixed-weight recurrent neural network as a function of the value(s) of said at least one external input signal and said at least one feedback input, an adaptive neural system having an cost input, an output and a plurality of nodes with variable weight interconnections between said nodes, said adaptive neural system output being coupled to at least one feedback input of said fixed-weight recurrent neural network to thereby vary a short-term memory of the fixed-weight recurrent neural network.