The general principle of a recognition system is the if…then relation: if event x in the recognisable domain, then event y in the recognition system. Recognition by neurological systems involves matching categorisables with neuronal activities that categorise. For perception, this means correlating difference (information) outside the system with difference (information) inside the system. The identity of any categorising activity within the system is given, therefore, by contrast to the other categorising activities within the system. Each categorising has no meaning without reference to other categorising.