Four Rules of Scientific Reasoning
from Principia Mathematica
by Isaac Newton

Sir Isaac Newton was a significant contributor to the Scientific Revolution. Newton believed that scientific theory should be coupled with rigorous experimentation, and he published four rules of scientific reasoning in Principia Mathematica (1686) that form part of modern approaches to science:
  1. admit no more causes of natural things than are both true and sufficient to explain their appearances,
  2. to the same natural effect, assign the same causes,
  3. qualities of bodies, which are found to belong to all bodies within experiments, are to be esteemed universal, and
  4. propositions collected from observation of phenomena should be viewed as accurate or very nearly true until contradicted by other phenomena.

Newton’s rules of scientific reasoning have proved remarkably enduring. His first rule is now commonly called the principle of parsimony, and states that the simplest explanation is generally the most likely. The second rule essentially means that special interpretations of data should not be used if a reasonable explanation already exists. The third rule suggests that explanations of phenomena determined through scientific investigation should apply to all instances of that phenomenon. Finally, the fourth rule lays the philosophical foundation of modern scientific theories, which are held to be true unless demonstrated otherwise. This is not to say that theories are accepted without evidence, nor that they can’t change – theories are built upon long lines of evidence, often from multiple pieces of research, and they are subject to change as that evidence grows.