"From the Phenomena of Motions to the Forces of Nature": Hypothesis or Deduction?

This paper examines Newton's argument from the phenomena to the law of universal gravitation-especially the question how such a result could have been obtained from the evidential base on which that argument rests. Its thesis is that the crucial step was a certain application of the third law of motion-one that could only be justified by appeal to the consequences of the resulting theory; and that the general concept of interaction embodied in Newton's use of the third law most probably evolved in the course of the very investigation that led to this theory.