The subject of the paper is Plotinus' theory of the 'One'. The motif to work on the issue was the need of a systematic treatment of this problem, still missing in the writings of Slovak historians of philosophy. The intention of the paper is the presentation, analysis and critical interpretation of the remarkable origins of Plotinus' theory. The paper deals further with the question of the sources of Plotinus' theory of the 'One' in Plato. The introductory general theoretical reflections on the origins of Plotinus' theory (the first part of the paper) is followed by a detailed text analysis of the books 6 and 7 of Plato's Republic (the second part) and by the analysis of the first and second hypotheses of the second part of the dialogue Parmenides (the third part of the paper).