The overall aim of this book is to explain certain forms of scientific success. One set of theories that exhibit all three of the forms of success with which we are here concerned is Isaac Newton's three laws of motion and law of gravitation. Newton's laws have been used in the derivation of numerous novel predictions, including the derivation of the prediction of the existence of the outer planets, their positions at a particular time, their orbit paths, their momenta and their departure from perfect sphericity. So Newton's laws clearly exhibit the first phenomenon. As we have already observed, they have also led us to a knowledge of some parts of reality, such as the planet Neptune, that were not accessible at the time they were first postulated, but whose existence is now beyond serious dispute. So they exemplify the second of our phenomena. Finally, at least some of Newton's laws are, arguably, close to a priori, and so exhibit the third phenomenon. Since Newton's laws exemplify all three of the types of success with which we are here concerned, it is highly desirable that the account advocated here be able to explain the successes of those laws.

NEWTON'S THREE LAWS OF MOTION

Newton's three laws of motion (Newton 1960: 13) are:

1. Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.

2. The change of motion is proportional to the motive force impressed and is made in the direction of the straight line in which that force is impressed.

3. To every action there is always opposed an equal reaction, or the mutual actions of two bodies upon each other are always equal and directed to contrary parts.