Trying to understand the functions of any part of the brain two questions may be asked: "What is it doing?" and "How is it doing it?" Usually these questions are answered simultaneously in terms of a postulated network of interconnected neurons. There is a tendency to formulate all kind of descriptions in brain science only using structural concepts. This reflects the generally adopted idea of the neuron as the functional building-brick of the brain. From a more general point of view, however, it turns out that the two questions sometimes ought to be answered separately, starting with the first one. This conclusion relies upon a study of the basis for neurophysiological theory-making, specially the interaction between observations and hypotheses. Epistemological considerations as well as scrutiny of the available experimental results lead to three basic theses, one on neural support, one on the functional potential of neural systems and one on the definition of neural signals: The general conclusion, stated as a thesis on neural systems, is that with the special kind of experimental results usually available so far, it may often be better to use only hypotheses on the total behaviour of a neural subsystem, disregarding the details of the network producing that behaviour. We call this kind of description on a certain phenomenological level "neurodynamics" to stress that it is a concept different from other kinds of brain theories. Using it we may define the meaning of"information" in a neural system when referring to the content and not the amount. There seems to be a trend towards the neurodynamic approach, but an extensive and consequent' use of its principles will have to wait for more powerful experimental techniques.