Main Difference
The main difference between Principle and Theory is that the Principle is a rule that has to be followed or is an inevitable consequence of something, such as the laws observed in nature and Theory is a contemplative and rational type of abstract or generalizing thinking, or the results of such thinking.
Principle
A principle is a concept or value that is a guide for behavior or evaluation. In law, it is a rule that has to be, or usually is to be followed, or can be desirably followed, or is an inevitable consequence of something, such as the laws observed in nature or the way that a system is constructed. The principles of such a system are understood by its users as the essential characteristics of the system, or reflecting system’s designed purpose, and the effective operation or use of which would be impossible if any one of the principles was to be ignored. A system may be explicitly based on and implemented from a document of principles as was done in IBM’s 360/370 Principles of Operation.
Examples of principles are, entropy in a number of fields, least action in physics, those in descriptive comprehensive and fundamental law: doctrines or assumptions forming normative rules of conduct, separation of church and state in statecraft, the central dogma of molecular biology, fairness in ethics, etc.
In common English, it is a substantive and collective term referring to rule governance, the absence of which, being “unprincipled”, is considered a character defect. It may also be used to declare that a reality has diverged from some ideal or norm as when something is said to be true only “in principle” but not in fact.
Theory
A theory is a contemplative and rational type of abstract or generalizing thinking, or the results of such thinking. Depending on the context, the results might, for example, include generalized explanations of how nature works. The word has its roots in ancient Greek, but in modern use it has taken on several related meanings.
Theories guide the enterprise of finding facts rather than of reaching goals, and are neutral concerning alternatives among values. A theory can be a body of knowledge, which may or may not be associated with particular explanatory models. To theorize is to develop this body of knowledge.
As already in Aristotle’s definitions, theory is very often contrasted to “practice” (from Greek praxis, πρᾶξις) a Greek term for doing, which is opposed to theory because pure theory involves no doing apart from itself. A classical example of the distinction between “theoretical” and “practical” uses the discipline of medicine: medical theory involves trying to understand the causes and nature of health and sickness, while the practical side of medicine is trying to make people healthy. These two things are related but can be independent, because it is possible to research health and sickness without curing specific patients, and it is possible to cure a patient without knowing how the cure worked.
In modern science, the term “theory” refers to scientific theories, a well-confirmed type of explanation of nature, made in a way consistent with scientific method, and fulfilling the criteria required by modern science. Such theories are described in such a way that any scientist in the field is in a position to understand and either provide empirical support (“verify”) or empirically contradict (“falsify”) it. Scientific theories are the most reliable, rigorous, and comprehensive form of scientific knowledge, in contrast to more common uses of the word “theory” that imply that something is unproven or speculative (which is better characterized by the word hypothesis). Scientific theories are distinguished from hypotheses, which are individual empirically testable conjectures, and from scientific laws, which are descriptive accounts of how nature behaves under certain conditions.
Principle (noun)
A fundamental assumption or guiding belief.
“We need some sort of principles to reason from.”
Principle (noun)
A rule used to choose among solutions to a problem.
“The principle of least privilege holds that a process should only receive the permissions it needs.”
Principle (noun)
Moral rule or aspect.
“I don’t doubt your principles.”
“You are clearly a person of principle.”
“It’s the principle of the thing; I won’t do business with someone I can’t trust.”
Principle (noun)
A rule or law of nature, or the basic idea on how the laws of nature are applied.
“Bernoulli’s Principle”
“The Pauli Exclusion Principle prevents two fermions from occupying the same state.”
“The principle of the internal combustion engine”
Principle (noun)
A fundamental essence, particularly one producing a given quality.
“Many believe that life is the result of some vital principle.”
Principle (noun)
A beginning.
Principle (noun)
A source, or origin; that from which anything proceeds; fundamental substance or energy; primordial substance; ultimate element, or cause.
Principle (noun)
An original faculty or endowment.
Principle (verb)
To equip with principles; to establish, or fix, in certain principles; to impress with any tenet or rule of conduct.
Theory (noun)
Mental conception; reflection, consideration. 16th-18th c.
Theory (noun)
A phenomena and correctly predicts new facts or phenomena not previously observed, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc. from 17th c.
Theory (noun)
The underlying principles or methods of a given technical skill, art etc., as opposed to its practice. from 17th c.
Theory (noun)
A field of study attempting to exhaustively describe a particular class of constructs. from 18th c.
“Knot theory classifies the mappings of a circle into 3-space.”
Theory (noun)
A hypothesis or conjecture. from 18th c.
Theory (noun)
A set of axioms together with all statements derivable from them. Equivalently, a formal language plus a set of axioms (from which can then be derived theorems).
“A theory is consistent if it has a model.”
Principle (noun)
a fundamental truth or proposition that serves as the foundation for a system of belief or behaviour or for a chain of reasoning
“the basic principles of justice”
Principle (noun)
a rule or belief governing one’s behaviour
“she resigned over a matter of principle”
“struggling to be true to their own principles”
Principle (noun)
morally correct behaviour and attitudes
“a man of principle”
Principle (noun)
a general scientific theorem or law that has numerous special applications across a wide field.
Principle (noun)
a natural law forming the basis for the construction or working of a machine
“these machines all operate on the same general principle”
Principle (noun)
a fundamental source or basis of something
“the first principle of all things was water”
Principle (noun)
a fundamental quality determining the nature of something
“the combination of male and female principles”
Principle (noun)
an active or characteristic constituent of a substance, obtained by simple analysis or separation
“the active principle of Spanish fly”