Nature of Scientific Theories


Scientific Theory

  • A scientific theory is an axiom system
    • designed to explain certain kinds of phenomena
    • defined by its postulates
    • supported or disproved by its predictions
  • A scientific theory is not an amorphous collection of ideas.

Axiom Systems

  • An axiom system is a set of axioms (or postulates) from which theorems (or predictions) are logically derived.
  • The classic example is the system of Plane Geometry in Euclid’s Elements, consisting of: 
    • Axioms / Postulates
    • Theorems
    • Proofs of Theorems.
English translation of the Elements 1570

View Axiom System


Scientific Theories are designed to explain certain kinds of phenomena

  • Newton’s Theory of Gravitation
    • Orbits of heavenly bodies
    • Motion of falling bodies
  • Mendel’s Theory of Heredity
    • How certain characteristics of pea plants are transmitted to their offspring
  • Quantum Mechanics
    • Interactions of atoms and subatomic particles with one another and with electromagnetic radiation.
  • Einstein’s Special Relativity
    • Why the observed speed of light is the same, regardless of the relative motion sender and receiver.

View Explanation


Scientific theories are defined by their postulates.

  • Newton’s Theory of Gravitation
    • Law of Universal Gravitation
    • Equation of Motion
    • Law of Action and Reaction
  • Mendel’s Theory of Heredity
    • Determination Postulate
    • Inheritance Postulate
  • Darwin’s Theory of Evolution
    • Common Descent with Modification
  • Einstein’s General Relativity
    • Field Equation
    • Geodesic Postulate
  • Theory of Electromagnetism
    • Maxwell’s Equations
    • Lorentz Force Law
    • Equation of Motion
  • Quantum Mechanics
    • State/Vector Representation
    • Observable/Operator Representation
    • Time Evolution (Schrodinger Equation)
    • Prediction
    • Collapse
  • Einstein’s Special Relativity
    • Principle of Relativity
    • Constancy of Speed of Light
Postulates of Newton’s Theory of Gravitation
  • Law of Universal Gravitation
    • Informal: Things with mass attract each other
    • Precise: For any pair of physical bodies there’s a force on each, toward the other, with magnitude Gm1m2/r2
      • where G = 6.67384 10-11 (the gravitational constant), m1 and m2 are the masses of the bodies, and r is the distance between them
    • Formula: F = Gm1m2/r2
  • Equation of Motion
    • Informal: Forces makes things move
    • Precise: The acceleration A a physical body equals the net force F on the body divided by its mass M
      • Acceleration is the rate of change of velocity
      • Formula: F = MA
  • Action and Reaction
    • Forces come in pairs, equal in magnitude and opposite in direction


A scientific theory is supported or disproved by its predictions

A single refuted prediction disproves a theory
  • A theory is disproved by the valid deductive argument-form modus tollens
    • If T then P
    • It’s false that P
    • Therefore, it’s false that T
  • Example of modus tollens
    • If 2019 is a leap year, it’s (evenly) divisible by 4.
    • 2019 is not divisible by 4.
    • Therefore, 2019 is not a leap year.
  • Disproof of Newton’s Theory of Gravitation
    • If Newton’s theory is true, Mercury’s orbit precesses 5557 seconds of arc per century
    • Mercury’s orbit does not precess 5557 seconds of arc per century.
    • Therefore, Newton’s theory is false.
Confirmed predictions support a theory
  • A scientific theory is supported by its confirmed predictions.
  • The more unexpected the truth of prediction (apart from the theory), the greater the support.  
    • Unexpected predictions: 
      • Electromagnetism’s prediction of Radio Waves
      • Special Relativity’s prediction of Time Dilation
      • Relativistic Quantum Mechanics’ prediction of Antimatter
      • Big Bang Theory’s prediction of CMB Radiation
      • General Relativity’s prediction of Gravitational Waves
  • The greater the variety of verified predictions, the greater the support.
    • General Relativity’s prediction of
      • Orbits of Heavenly Bodies, Motion of Falling Bodies, Time Dilation, Gravitational Redshift, Gravity Waves, Deflection of Light

View Hypothetico-deductive Support

Famous Scientific Predictions
  • John Couch Adams in 1843 and Urbain Le Verrier in 1845 independently predicted the existence of Neptune, based on Newton’s theory of gravitation and the wayward orbit of Uranus.  Neptune was observed in 1846.
  • James Clerk Maxwell in 1873 predicted the existence of electromagnetic waves, based on his theory of electromagnetism.   Heinrich Hertz used Maxwell’s equations to generate radio waves in 1887.
  • Albert Einstein in 1915 predicted gravitational time dilation, based on his theory of General Relativity.  Hafele and Keating verified the prediction in 1971 using atomic clocks on board airplanes.
  • Wolfgang Pauli in 1930 predicted the existence of the neutrino, based on the law of the conservation of energy and the process of beta decay.  The neutrino was detected in 1956.
  • Paul Dirac in 1931 predicted the existence of antimatter, based on his theory of relativistic quantum mechanics.  Carl Anderson discovered the positron in 1932.
  • Ralph Alpher and Robert Herman in 1948 predicted the existence of CMB Radiation, based on the Big Bang Theory.  Arno Penzias and Robert Wilson detected the CMB radiation in 1964. 
  • In 1964 Peter Higgs and others predicted the Higgs Boson based on the Standard Model of Particle Physics. The particle was detected at the Large Hadron Collider at CERN in 2012


Newton’s Theory of Gravitation

View Newton’s Theory of Gravitation

Mendel’s Theory of Heredity

View Mendel’s Theory of Heredity

Einstein’s Special Relativity

View Special Relativity



  • Measurement associates numbers with physical quantities: length, mass, volume, time, electric charge, velocity, location, distance, etc.
  • Physics uses the International System of Units (SI), where:
  • Measurements are made relative to a reference frame, a system of abstract space and time coordinates imposed on a latticework of measuring rods and synchronized clocks.

Principle of Simplicity

  • Principle of Simplicity
    • The simpler theory is more likely to be true, other things being equal.
  • Occam’s Razor (Ockham’s Razor)
    • “Entities should not be multiplied beyond necessity.”
  • Newton’s Law of Parsimony
    • “We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances.”
  • Einstein
    • “The grand aim of science is to cover the greatest number of empirical facts by logical deduction from the smallest number of hypotheses or axioms.”
      • That is, the objective of a scientific theory is to predict the greatest range of empirical facts by deriving them from the smallest, simplest set of postulates
      • A scientific theory should predict the mostest from the leastist.

Mathematical and Scientific Theories

  • Mathematical and scientific theories are both axiom systems.
  • They differ in their subject matter: abstract entities vs physical phenomena.
  • Theory of Arithmetic
    • Axioms
      • Zero is a natural number.
      • Every natural number has a successor in the natural numbers.
      • Zero is not the successor of any natural number.
      • If the successor of two natural numbers is the same, then the two original numbers are the same.
      • If a set contains zero and the successor of every number is in the set, then the set contains the natural numbers.
    • Theorems (logically derived from axioms)
      • For any natural numbers x, y, and z, x · (y + z) = (x · y) + (x · z)
  • Newton’s Theory of Gravitation
    • Postulates
      • Law of Universal Gravitation
      • Equation of Motion
      • Action and Reaction
    • Predictions and Laws (logically derived from postulates)
      • Kepler’s Laws of Planetary Motion

Scientific Laws and Theories

  • A scientific law (or law of nature) is an empirical generalization that doesn’t just happen to be true, like the accidental generalization that all gold spheres are less than one mile in diameter. 
  • Differences between Laws and Theories
    • A law must be true, whereas a theory can be false. If what was thought to be a law is disproven, it was never a law.  A disproved theory stays a theory.
    • A theory may have more than one postulate but a law is a single proposition.
  • Some laws are postulates of a theory
    • For example, the Law of Universal Gravitation is a postulate of Newton’s Theory of Gravitation
  • Some laws are generalizations derived from the postulates of a theory
    • For example Kepler’s Laws of Planetary Motion are derived from Newton’s Theory.
  • Principle is another name for Law
    • For example, the Heisenberg Uncertainty Principle is derived from the postulates of Quantum Mechanics.

Hypotheses, Conjectures, and Theories

  • A hypothesis is a “a tentative assumption made in order to draw out and test its logical or empirical consequences” (Merriam-Webster)
  • A conjecture is a “hypothesis that has been formed by speculating, usually with little hard evidence.” (Wordnet)
  • A theory typically begins as a conjecture or hypothesis.  As evidence accumulates, it ceases being a hypothesis and becomes a well-confirmed theory.  It’s an established fact if the evidence becomes overwhelming.

Unscientific Hypotheses

  • Unscientific hypotheses are untestable:
    • Everything that happens is God’s will
    • People have souls
    • God created life
    • A one-celled human zygote is a human being
    • Nothing is known with certainty
    • Doctrine of Transubstantiation

How Scientific Theories Developed

  • In the 5th century BC the Greeks developed theories of nature, for example:
    • Empedocles (5th Century BC)
    • The theories generated after-the-fact explanations but no testable predictions
  • In 1623 Galileo wrote that:
    • “The book of nature is written in the language of mathematics.”
  • In 1687 Newton set forth his theory of gravitation, which made precise, testable predictions (thanks to the magic of differential equations).

Dictionary Senses of Theory

Dictionary Senses of Fact

Two Sides of Science

  • Empirical Side
    • Observation
      • Passive: Investigator doesn’t control observational conditions, e.g. astronomy
    • Experiment
      • Active: Investigator has control of experimental conditions, e.g. physics
    • Use of Math
      • Counting objects, e.g. pea plants with yellow seeds
      • Measuring quantities, e.g. time, length, distance, velocity, mass, electric charge
  • Theoretical Side
    • Theory
      • System of postulates from which predictions are derived.
    • Use of Math
      • Formulating the postulates of a theory
      • Deriving predictions from a theory