Fred asked:

When Newton said, ‘Hypotheses non fingo’ was he lying?

Answer by Helier Robinson

No, probably not: he almost certainly believed that he was being exclusively empirical. However, his first law — a body remains at rest or in uniform motion unless acted upon by an outside force — is hypothetical, speculative: it could only be tested empirically by going into space, far from any gravitational fields.


Answer by Geoffrey Klempner

One of Isaac Newton’s greatest fans, Immanuel Kant, has a nice take on Newton’s claim in the Preface to the 1st edition of the Critique of Pure Reason:

"As to certainty, I have prescribed to myself the maxim, that in this kind of investigation it is in no wise permissible to hold opinions. Everything, therefore, which bears any manner of resemblance to an hypothesis is to be treated as contraband; it is not to be put up for sale even at the lowest price, but forthwith confiscated, immediately upon detection.”

Surely, there’s a huge difference between Kant’s investigation and Newton’s? Kant is doing philosophy, and his conclusions are intended as necessary truths. Newton is doing empirical science.

This is an over-simplification. Newton’s First Law of motion is, in fact, an instantiation of Leibniz’s Principle of Sufficient Reason, which is a necessary truth. There is no reason why a moving body would slow down or stop, unless a force acted upon it. It was Newton’s genius to see that what everyone observes to take place in the natural world — bodies ‘naturally’ slow down unless you keep pushing them along — has an explanation in terms of the First Law, because in our real world (discounting the far reaches of space) we always have to reckon with the forces of gravity and friction.

This is just one example of many where truths of reason are explicitly or implicitly appealed to in formulating physical theories. The theories can still turn out to be wrong. The alleged ‘truths of reason’ could themselves be incorrect or require radical revision. However, Newton was right in his faith that reason has a large part to play in discovering and formulating the laws of physics.