Controlling Idea:  The theme of a literary work.  The controlling idea of a poem is the idea continuously developed throughout the poem by sets of key words that identify the poet's subject and his attitude or feeling about it.  It may also be suggested by the title of a poem or by segment of the poem.  It is rarely stated explicitly by the poet, but it can be stated by the reader and it can be stated in different ways.  The controlling idea is an idea, not a moral; it is a major idea, not a minor supporting idea or detail; and it controls or dominates the poem as a whole. 
 

           The word theme is here used to name the particular subject matter of the poem in relationship to the reader's previous observation of the life about him and within him.  Theme, then, here refers to those broad generalizations and high-order abstractions which each person develops in dealing with the common experiences of life.  Each of us was born, and each of us will die.  And, then no  one of us can report his own birth of his own dearth, everyone had had some personal observation at first of second hand of the elemental and universal facts of life, Birth and Death.  So, too, every mature person has had some experience of what we shall call  of Heart of and Mind, of Friendship and of Love, of Youth and Of Nature and of Art, of Work and of Play, of War and of Justice, of Doubt and of Terror…; and most persons will add that they have had some experience of Faith and of God and is not  complete list of universal experiences, but it will do to suggest the possible range of poetic themes. 
 
 
 
 
 
 
 
 

Reference: Cooper, C. W., "Preface to Poetry", 1946. 
                 Millett, N. C., "How to read a poem", 1966.