The paper aims at a unification of the two directions in contemporary philosophy of science: the direction which deals with the relation of data to phenomena with the direction which deals with the knowledge about mechanism and its employment in scientific thinking. It aims also at a reconstruction of the development of scientific knowledge which is characterized in contemporary philosophy of science as a movement from data, via phenomena, to mechanisms. An attempt will be made to show that this in fact amounts to an assignment of philosophical categories like data, phenomena, mechanism, etc. This unification and reconstruction draws also on the reconstruction of the main stages of the development of knowledge leading from A.-J. Ångström’s measurement of the wave-lengths of spectral lines of hydrogen in 1868 to N. Bohr’s theory of the hydrogen atom proposed in 1913.