An objective correlative is a literary term referring to a symbolic article used to provide explicit, rather than implicit, access to such traditionally inexplicable concepts as emotion or colour.
Origin of terminology 
|“||Take an example from one of the lower forms of organic life,--a common vegetable. Will any one assert that the surrounding inorganic elements of air, earth, heat, and water produce its peculiar form? Though some, or all, of these may be essential to its development, they are so only as its predetermined correlatives, without which its existence could not be manifested; and in like manner must the peculiar form of the vegetable preexist in its life, — in its idea, — in order to evolve by these assimilants its own proper organism.
No possible modification in the degrees or proportion of these elements can change the specific form of a plant, — for instance, a cabbage into a cauliflower; it must ever remain a cabbage, small or large, good or bad. So, too, is the external world to the mind; which needs, also, as the condition of its manifestation, its objective correlative. Hence the presence of some outward object, predetermined to correspond to the preexisting idea in its living power, is essential to the evolution of its proper end, — the pleasurable emotion.
Eliot used the term exclusively to refer to his claimed artistic mechanism whereby emotion is evoked in the audience:
|“||The only way of expressing emotion in the form of art is by finding an "objective correlative"; in other words, a set of objects, a situation, a chain of events which shall be the formula of that particular emotion; such that when the external facts, which must terminate in sensory experience, are given, the emotion is immediately evoked.||”|
It seems to be in deference to this principle that Eliot famously described Hamlet as "most certainly an artistic failure": Eliot felt that Hamlet's strong emotions "exceeded the facts" of the play, which is to say they were not supported by an "objective correlative." He acknowledged that such a circumstance is "something which every person of sensibility has known"; but felt that in trying to represent it dramatically, "Shakespeare tackled a problem which proved too much for him."
Theory of the objective correlative 
The theory of the objective correlative as it relates to literature was largely developed through the writings of the poet and literary critic T.S. Eliot, who is associated with the literary group called the New Critics. Helping define the objective correlative, T.S. Eliot’s essay “Hamlet and His Problems”  in his book The Sacred Wood: Essays on Poetry and Criticism discusses his view of Shakespeare’s incomplete development of Hamlet’s emotions. In this essay, Eliot states: “The artistic ‘inevitability’ lies in this complete adequacy of the external to the emotion….”. According to Eliot, the feelings of Hamlet are not sufficiently supported by the story and the other characters surrounding him. The objective correlative’s purpose is to express the character’s emotions by showing rather than describing feelings as pictured earlier by Plato and referred to by Peter Barry in his book Beginning Theory: An Introduction to Literary and Cultural Theory as “…perhaps little more than the ancient distinction (first made by Plato) between mimesis and diegesis….” (28). According to Formalist critics, this action of creating an emotion through external factors and evidence linked together and thus forming an objective correlative should produce an author’s detachment from the depicted character and unite the emotion of the literary work.
Criticisms of the objective correlative 
One possible criticism of Eliot’s theory includes his assumption that an author’s intentions concerning expression will be understood in one way only. This point is stated by Balachandra Rajan as quoted in David A. Goldfarb’s “New Reference Works in Literary Theory”  with these words: “Eliot argues that there is a verbal formula for any given state of emotion which, when found and used, will evoke that state and no other.”
- Barry, Peter: Beginning Theory. An Introduction to Literary and Cultural Theory. 2nd ed. New York: Manchester University Press, 2002.
- Eliot, T. S. “Hamlet and His Problems.” 5 April. 2007. http://www.bartleby.com/200/sw9.html.
- Eliseo Vivas, The Objective Correlative of T. S. Eliot, reprinted in Critiques and Essays in Criticism, ed. Robert W. Stallman (1949).
- Goldfarb, David A. “New Reference Works in Literary Theory.” Conference: a journal of philosophy and theory, 1995. 9 April 2007. http://www.echonyc.com/~goldfarb/encyc.htm.
- Heehler, Tom. The Well-Spoken Thesaurus: The Objective Correlative and Barbara Kingsolver. Sourcebooks, 2011.
- Witkoski, Michael. “The bottle that isn’t there and the duck that can’t be heard: The ‘subjective correlative’ in commercial messages.” Studies in Media & Information Literacy Education. Vol. 3. Toronto: Toronto Press, 2003. 9 April 2007. http://www.utpjournals.com/simile/issue11/witkoskifulltext.html.
- http://www.bartleby.com/200/sw9.html "Hamlet and His Problems"
- http://www.gutenberg.org/files/11391/11391.txt Lectures on Art