Intertexuality

To understand what intertextuality is, one has to understand what a text is.

Because it has language as its raw material, the text is thus a linguistic product.

Language is a system of signs. This system is primarily conventional and agreed upon by a group. Because language is a social phenomenon, using language implies accepting and reproducing conventional codes as well as adopting established linguistic standards and norms. Language, because of its social nature, cannot be invented out of the blue and cannot exist in a vacuum. Language cannot be isolated from its social context. Language cannot be individual. When we use language we are not imitating or being influenced by anybody in particular. We are simply using a code which is a common property (it does not belong to any particular individual). There is no copyright infringement when using language.   

Has language been independent, individual and isolated, it would fail its communicative function. 

The act of communication as described by Jakobson demonstrate that for the message to be understood and for communication to take place, the code has to be common and shared between both the sender and the receiver. If the code is individual, it would be impossible for the receiver to crack it. The code should be part of the established standards - part of the norms. 

                      Context

                           ^

                           ^

Sender  >>>  Message  >>>  Receiver

                           ^

                           ^

                     Channel

                           ^

                           ^

                       Code

Even new codes, they always build up on already established codes (or else it would be impossible for them to be established in themselves as codes). This means that meaning is never independent.

Text and language can be used interchangeably. What stands for language stands for the text. 

A literary text is the construct of literary systems, codes, and traditions established by previous literary texts. For instance, there is no copyright infringement in using the expression "Once upon a time" at the beginning of a fairy tale or of the act of story telling because the expression has come to be a code as well as a cultural, a literary and a social property. Its use carries literary, cultural and social meanings for the reader. Every text is part of a literary tradition. Even texts reacting against a given literary tradition are compelled to use the existing literary codes to react against that tradition. For instance, James Joyce in his A Portrait of the Artist as Young Man reacts against traditions. However, he starts his novel with "Once upon a time". It is used as a sort of hint to that against which the text reacts.   

In that respect there is no such thing as 'source text' or 'original text'. Every text is an intertext because every text builds up on previous texts. Every text constructs its meaning only in relation to previous texts. No text carries its proper  independent meaning. No text is hermetically closed and self-sufficient. No text exists in a vacuum. No text is isolated. No text is original and unique in itself. 

The text's hermeneutics and its potential meanings can only be discovered by tracing the relations existing between that text and other texts that has gone before it. Meaning exists between a text and all the other texts to which it refers and relates. For such a reason, the meaning of a text can never be exhausted and stabilized. Ceaselessly new textual relations are discovered / created / possible:

According to Roland Barthes literary meaning can never be stabilized by the reader, since the literary work’s intertextual nature always leads reader on to new textual relations. As Barthes reminds us, the very word ‘text’ is, if we remember its original meanings, «a tissue, a woven fabric»: «We know now that a text is not a line of words releasing a single ‘theological’ meaning [...] but a multidimensional space in which a variety of writings, none of them original, blend and clash. The text is a tissue of quotations drawn from the innumerable centers of culture...» 

Definition from the Dictionary of Literary Terms and Literary Theory:

A term coined by Julia Kristeva in 1966 to denote the interdependence of literary texts, the interdependence of any one literary text with all those that have gone before it. Her contention was that a literary text is not an isolated phenomenon but is made up of a mosaic of quotations, and that any text is the 'absorption and transformation of another'. she challenges traditional notions of literary influence, saying that intertextuality denotes a transposition of one or several sign systems into another or others. But this is not connected with the study of sources. 'Transposition' is a Freudian term, and Kristeva is pointing not merely to the way texts echo each other but to the way that discourses or sign systems are transposed into one another - so that meanings in one kind of discourse are overlaid with meanings from another kind of discourse. It is a kind of 'new articulation'. P424

Roland Barthes and the Death of the Author:

There are two axes to intertextuality:

As far as the author is concerned: The author, before being an author, is a reader. He, consciously or unconsciously brings his readings into his writing act. The writer reproduces, rewrites and renews old texts. There is no source text and thus no authorship. 

Any text is a new tissue of past citations. Bits of code, formulae, rhythmic models, fragments of social languages, etc., pass into the text and are redistributed within it, for there is always language before and around the text. Intertextuality, the condition of any text whatsoever, cannot, of course, be reduced to a problem of sources or influences; the intertext is a general field of anonymous formulae whose origin can scarcely ever be located; of unconscious or automatic quotations, given without quotation marks. ("Theory of the Text" 39).

As far as the reader is concerned: The text is what the reader makes it. In his act of reading, the reader establishes links between the text he is reading and other texts he has read in the past to give meaning to that text. The more links are established, the more the text is opened on and related to other texts, the richer in meaning it will be. 

It is not in the hand of the author to control how the reader will perceive his text. What counts is the point of view of the reader. The text belongs to a literary tradition as defined by the reader's reading background. The meanings that the reader will come to are forcefully dictated by his literary culture or his reading network.