Decentering is the essential concept of Derrida's Dissemination. The origin of what we define today as a post structural principle can be traced back to the ancient Greek philosophy, to Plato's defense of the "living spoken word" against the "cadaverous rigidity of writing". What Derrida does in his book is subverting Plato's general premise or simply reducing it to a circular reasoning, therefore to inconsequentiality.
Logos, a living, animate creature, is thus also an organism that has been engendered. An organism: a different body proper, with a center and extremities, joints, a head, and feet. In order to be "proper", a written discourse ought to submit to the laws of life just as a living discourse does. Logographical necessity [...] ought to be analogous to the biological, or rather zoological, necessity [...] Otherwise obviously it would have neither head nor tail. (79)
Of course there is a straight analogy between the spoken word and its supposed origin, the human body. But, at the same time, assuming that the written word is cadaverous is a false syllogism.
Sperm, water, ink, paint, perfumed dye: the pharmakon always penetrates like a liquid; it is absorbed, drunk, introduced into the inside, which it first marks with the hardness of the type, soon to invade it and inundate it with its medicine, its brew, its drink, its potion, its poison. (152)
Exactly because of the fact that the oral speech submits to biological rules, it is, like any form of written expression, uncontainable and fluid. What to Plato was a limit of the written word -- no head and no tail -- is actually the primary condition of the spoken word itself, of any form of communication.
The dissimulation of the woven texture can in any case take centuries to undo its web: a web that envelops a web, undoing the web for centuries; reconstructing it too as an organism, indefinitely regenerating its own tissue behind the cutting trace, the decision of each reading. (63, bold added)
This quotation leads us beyond the dichotomy of spoken/written word to the electronic environment: the web. It is true, and I find it amazing, that every new technology, like writing a long time ago, is somehow legitimated, or simply defined, by comparing it to the physical entity of the human body. And it is exactly out of the growing importance of biology in the last two centuries at least that we are talking of decentering. The attention has moved from an absent center, God, to a present "decenterer", the human being. The text is
Straightway plural, dived or multiplied, this is also because of its power of germination or germinal differentiation , which will proceed to engender or will have given birth to a whole chain of other sentences (338) There is always a sentence that has already been sealed somewhere waiting for you where you think you are opening a virgin territory (340, bold added)
Precisely because speech is a living entity, as Plato says, that it is intrinsically based on proliferation and reproduction, on engendering and multiplication. To me, decentering doesn't mean the "lack" of a center, but its mobility, flexibility and unsteadiness. The center is not a unifying principle anymore, but a variable. It reflects the differentiation in the democratic principle of life itself, where each individual is finally free to set himself as the center of his or her environment.