The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, θεωρία, meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action. Theory is especially often contrasted to "practice" (from Greek praxis, πρᾶξις) a Greek term for "doing", which is opposed to theory because theory involved no doing apart from itself.

A classical example of the distinction between theoretical and practical uses the discipline of medicine: Medical theory and theorizing 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.

By extension of the philosophical meaning, "theoria" is also a word still used in theological contexts.

In modern contexts, while theories in the arts and philosophy may address ideas and empirical phenomena which are not easily measurable, in modern science the term "theory", or "scientific theory" is generally understood to refer to a proposed explanation of empirical phenomena, made in a way consistent with scientific method. Such theories are preferably 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. In this modern scientific context the distinction between theory and practice corresponds roughly to the distinction between theoretical science and technology or applied science.

A common distinction made in science is between theories and hypotheses. Hypotheses are individual empirically testable conjectures, while theories are collections of hypotheses that are logically linked together into a coherent explanation of some aspect of reality and which have individually or jointly received some empirical support.

The differences between fact and theory and between theory and practice are a common concern of scientists, engineers, and philosophers.

• Fact is not enough, opinion is too much.
  • Todor Simeonov

• Facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts do not go away while scientists debate rival theories for explaining them. Einstein's theory of gravitation replaced Newton's, but apples did not suspend themselves in mid-air pending the outcome.
  • Stephen Jay Gould, 1981

• Facts are meaningless. You could use facts to prove anything that's even remotely true!
  • Homer Simpson

• In theory, there is no difference between theory and practice. But, in practice, there is.
  • Jan L. A. van de Snepscheut/Yogi Berra. Also cited in Applying Use Case Driven Object Modeling With UML, Doug Rosenberg and Kendall Scott p.1 [1]

• Facts were never pleasing to him. He acquired them with reluctance and got rid of them with relief. He was never on terms with them until he had stood them on their heads.
  • J.M. Barrie, The Greenwood Hat (1937)

• But facts are chiels that winna ding, and downa be disputed.
  • Robert Burns, A Dream (1786)

• There is no great harm in the theorist who makes up a new theory to fit a new event. But the theorist who starts with a false theory and then sees everything as making it come true is the most dangerous enemy of human reason.
  • Gilbert Keith Chesterton The Flying Inn

• "I should have more faith," he said; "I ought to know by this time that when a fact appears opposed to a long train of deductions it invariably proves to be capable of bearing some other interpretation."
  • Sir Arthur Conan Doyle, A Study in Scarlet (1887)
• It is a capital mistake to theorize before you have all the evidence. It biases the judgment.
  • Sir Arthur Conan Doyle, A Study in Scarlet (1887), Part 1, chap. 3
• It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.
  • Sir Arthur Conan Doyle, A Scandal in Bohemia (1891)

• Facts do not cease to exist because they are ignored.
  • Aldous Huxley, Proper Studies (1927)

• The fatal futility of Fact.
  • Henry James, Prefaces (1897)

• Talk to him of Jacob's ladder, and he would ask him the number of steps.
  • Douglas William Jerrold, Wit and Opinion of Douglas Jerrold (1895)

• A myth is, of course, not a fairy story. It is the presentation of facts belonging to one category in the idioms appropriate to another. To explode a myth is accordingly not to deny the facts but to re-allocate them.
  • Gilbert Ryle, The Concept of Mind (1949)

• Matters of fact, which as Mr Budgell somewhere observes, are very stubborn things.
  • Matthew Tindal, The Will of Matthew Tindal (1733)

• The ink of the scholar is more holy than the blood of the martyr.
  • said by Prophet Muhammad, founder of the religion of Islam