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. (shrink)