Were Ito deduceanysystem frommy feelings on leaving Eton, it might be called TheTheory of Permanent Adolescence.

-Connolly, Cyril Vernon
  Enemies of Promise, ch.24.

Read no history: nothing but biography, for that is life without theory.

-Disraeli, Benjamin, 1st Earl of Beaconsfield
  Contarini Fleming, pt.1, ch.23.

Grau, teurer Freund, ist alleTheorie. Und gru« n  des Lebens goldner Baum. All theory, dear friend, isgrey, but the golden tree of actual life springs ever green.

-Goethe,JohannWolfgang von
  Faust, pt.1,'Studierzimmer'.

An economist is someone who, when he finds something which works in practice, wonders if it will work in theory.

-Heller,Walter Wolfgang
   Attributed.

First, you know, a new theory is attacked as absurd; then it is admitted to be true, but obvious and insignificant; finally it is seen to be so important that its adversaries claim that they themselves discovered it.

-James,William
  Pragmatism, lecture 6.

Theory is a good thing but a good experiment lasts forever.

-Kapitsa, Peter Leonidovich
  Experiment, Theory, Practice.

Wie manches wu«  rde in derTheorie unwidersprechlich scheinen, wenn es dem Genie nicht gelungen w a« re, das Widerspiel durch dieTat zu erweisen. How many things would have appeared incontestable in theory if genius had not proved them wrong in practice. Levant

-Lessing, Gotthold Ephraim
  Laokoon: an essay upon the limits of painting and poetry, pt.4.

   If you wanttoknow thetaste of a pear, you musttastethe pear by eating it for yourself. If you want to know the theoryand methods of revolution, you must take part in revolution. All genuine knowledge originates in direct experience.

-Mao Zedong or MaoTse-tung
   Address to the  Anti- Japanese Military and Political College,  Jul.

   ThetheoryoftheCommunistsmay be summedup inthe single sentence: Abolition of private property.

-Marx, Karl Heinrich
  The Communist Manifesto (with Friedrich Engels, translated by Samuel Moore,1888).

Appeals to rationality are mostly bluff. There is no good theory of what it is nor of how to recognize it.

-Mellor, D H
  'Objective Decision Making', in Social Theory and Practice, Summer^Fall.

Democracy isthetheory thatthe commonpeopleknow what they want, and deserve to get it good and hard.

-Mencken, H(enry) L(ouis)
  Little Book in C Major.

We must respect the other fellow's religion, but only in the sense and to the extent that we respect his theory that his wife is beautiful and his children smart.

-Mencken, H(enry) L(ouis)
'Minority Report'. Collected in Notebooks (1956).

Happily, there is nothing in the laws of value which remains for the present or any future writer to clear up; the theory of the subject is complete.

-Mill,John Stuart
  Principles of Political Economy, with Some Applications to Social Philosophy.

The theory of our modern technic shows that nothing is as practical as the theory.

-Oppenheimer,J(ulius) Robert
  In Reflex,  Jul.

In order that a new theory should constitute a discovery or step forward it should conflict with its predecessor †it should contradict its predecessor; it should overthrow it. In this sense, progress in scienceöor at least striking progressöis always revolutionary.

-Popper, Sir Karl Raimund
  The Logic of Scientific Discovery.

The belief that we can start with pure observations alone, without anything in the nature of a theory, is absurd.

-Popper, Sir Karl Raimund
  Conjectures and Refutations (published1963), ch.1.

It is a fatal defect of current principles of administration that, like proverbs, they occur in pairs. For almost every principle one can find an equally plausible and acceptable contradictory principle. Although the two principles of the pair will lead to exactly opposite recommendations, there is nothing in the theory to indicate which is the appropriate one to apply.

-Simon, Herbert A
  Administrative Behavior.

The question of how to apply social theory to historical materials, as it is usually posed, is ridiculous.One does not apply theory to history; rather one uses history to develop theory.

-Stinchcombe, Arthur S
  Theoretical Methods in Social History, p.1.

Scientists have odious manners, except when you prop up their theory; then you can borrow money off them.

-Twain, Mark pseudonym of  Samuel Langhorne Clemens
The Bee (published1917).

The power of Christianity lies in its revelation in act, of that which Plato divined in theory.

-Whitehead, Alfred North
  Adventures of Ideas.