The paper understands methodological phenomenalism as the principle of focusing the scientific research exclusively on phenomena as the sole premises of scientific conclusions and theories. The principles according to which these conclusions are infered can be of either rational or empirical nature. Also, the very object of a scientific theory can either be pure fiction or true reality. The point is that the premises from which a scientific theory is derived are phenomena only. The principle of methodological phenomenalism was clearly and concisely expressed by Newton, in the words »... hypotheses non fingo«. The first part of the paper deals with Newton’s methodological phenomenalism. The second part deals with Bošković’s phenomenalism, most clearly and directly expounded in Adnotationes I, pp. 50–55, and briefly in Disquisitio in universam astronomiam, No. 1, p. 5. However, it can easily be discerned in Bošković’s definitions of the fundamental natural-scientific concepts – the force of inertia (vis inertiae) and the attractive and repulsive forces of potencies. The third part of the paper considers the Newtonian origin of Bošković’s phenomenalism, and the fourth examines phenomenalism as a constant of Bošković natural-scientific investigations, that is, as a principle which is found – in the same form – in both earlier and later Bošković’s physical treatises.