How did Newton mange to make the ground breaking discoveries he made in his lifetime? An insight into Newton's method is afforded by an examination of "inference rules" he devised.
We may still benefit from carefully considering these "rules.
" In the first edition of his "Principia" only the first two of his four rules appeared.
Rule III was published for the first time in the second edition of "Principia," while Rule III first appeared in the third edition.
It would appear, therefore, that Newton developed his four "inference rules" by reflecting on his inferential method in the course of the development of his thoughts on motion and gravity.
Physicists and philosophers to this day consider Newton's four "inference rules of philosophy" the first significant step to modern philosophy of physics.
We shall sketch the rules: Rule I: One should not allow more causes of natural phenomena than are true and sufficient to explain them.
Rule II: That to natural effects of the same kind one should endeavor as far as possible to allow the same causes.
Rule III: The properties of bodies which do not intensify or weaken (wax or wane) and belong to every body we have observed should be taken as properties of all bodies.
Rule IV: In experimental science, any proposition derived by induction from phenomena should be held to be true, either exactly or by nearest approximation, regardless of any other hypothesis until other observations are made which allow for more accurate inductions or explain exceptions to the first.
Both Rules I and II deal with the assignment of "causes" to "effects.
" There is a difference in the notion of "cause" and "effect" as understood in Newtonian thought from its conception in Humean philosophy.
A cause in the Humean understanding is defined with regard to the fact of its preceding its effect or on other terms its contiguity to its effect.
The conditions set in Humean philosophy are, however, not met in Newtonian gravitational phenomenon, for gravity cannot be described as preceding acceleration nor is it contiguous with it.
Newton seemed to have taken the concept of "cause" in the Aristotelian sense of being anything which helps or contributes to explaining another phenomenon.
Rule I has been interpreted as exhorting us to stop further enquiry when we have "sufficient" explanation of the phenomenon under investigation.
This rule is a rule of economy with regard to causes and their effects.
Where we have an explanation of cause which appears sufficient for an effect, further enquiry may only lead to dredging up false causes and may prove altogether misleading.
Rule II follows from Rule I and was derived ultimately from Ockham's rule of minimal inference which says that we must assign a single class of causes to effects of the same kind.
This rule is easier to state than follow for it assumes that we will encounter no problems in classifying causes and effects.
Newton, for instance, may be considered to have applied his imagination creatively in stating his rules of motion with regard to acceleration when he stated that acceleration is the same for all heavenly bodies under the same condition of force.
Rule III constrains us to consider any property of a body as common to all other bodies if we find the property as applying to all bodies we have been able to examine.
Rule IV acknowledges that we may in the long run discover exceptions, but pending such discovery Rule III should be adhered to.
Newton's rules of inference have their difficulties in practical application for it takes much for granted, as in the problem of categorizing causes and effect into same or different classes, and of deciding when a cause explanation of an effect may be considered "sufficient.
"