The paper deals with Newton’s eight definitions from his Principia making them a subject of logical-semantic and epistemological analyses. First, it lists these definitions and then presents two views on the nature of definition, as given in recent scholarly works. These views are applied to Newton’s definitions. Resulting from this application is the conclusion that Transparent Intensional Logic’s approach to definitions, once the latter contain magnitudes, is unable to reconstruct the fact that the magnitude (or magnitudes) in the definiens is (are) different from the magnitude in the definiendum. Another result is the recognition that Newton’s eight definitions, regardless of their a priori nature, still yield an increase of knowledge about the world. This conclusion is justified by Newton’s computation of the mass of planets and his reflection on the possibility of space flight.