Here is a favourite passage of mine from Johns Stuart Mill autobiography (he is speaking of the demanding education his father arranged for him): I remember at

Here is a favourite passage of mine from Johns Stuart Mill autobiography (he is speaking of the demanding education his father arranged for him): I remember at some time in my thirteenth year, on my happening to use the word idea, he asked me what an idea was; and expressed some displeasure at my ineffectual efforts to define the word: I recollect also his indignation at my using the common expression that something was true in theory but required correction in practice; and how, after making me vainly strive to define the word theory, he explained its meaning, and showed the fallacy of the vulgar form of speech which I had used; leaving me fully persuaded that in being unable to give a correct definition of Theory, and in speaking of it as something which might be at variance with practice, I had shown unparalleled ignorance. In this he seems, and perhaps was, very unreasonable; but I think, only in being angry at my failure. A pupil from whom nothing is ever demanded which he cannot do, never does all he can.