1. A neural network system comprising: qualitative evaluation means for qualitatively analyzing unknown data supplied thereto and normalizing the qualitatively analyzed unknown data within a predetermined range; computing means, including a neural network having a plurality of neurons, for computing desired data from the normalized unknown data from said first qualitative evaluation means, each neuron being connected to other neurons through synapses, each synapse being given a weight, each neuron outputting a value of an output function given to the neuron in accordance with a total sum of product values of output function values from said connected neurons and said synapse weights, wherein said computing means further includes: means for storing a causal network representing qualitative causal relation between the unknown data and the desired data, and network defining means for defining said neural network by connecting neurons through synapses in accordance with the causal network; and quantitative evaluation means for quantitatively analyzing desired data received from said computing means and outputting the quantitatively analyzed desired data.