Eliot argues for what he calls the "impersonality" of poetry. For Eliot, "poetry" is a living thing, a tradition that is both past and present which, in the West, stretches back to Homer. Poetry should be measured, he argues, not by its ability to express the personality of the poet but by its connection to the tradition from which it emerges. Great poetry exists in dialog with the poetry that came before it:
No poet...has his complete meaning alone. His significance...is the appreciation of his relation to the dead poets and artists.
In this way, Eliot seeks to separate poetry as an object in itself from the personality of the poet who created it. In fact, the artistic development of the poet is understood as progress toward the "extinction of personality," a kind of "self-sacrifice" for one's art. Instead of reflecting her own personality, the work of the mature poet lies in their ability to create new combinations of poetic effects or feelings based on the relation of her art to that of the "tradition."
Eliot means this in a specific, even scientific, way. He uses an analogy from chemistry, the reaction of platinum to a catalyst of oxygen and sulphuric dioxide: in Eliot's formulation, the platinum is akin to the "mind of the poet," which transmutes the oxygen and sulphuric dioxide into sulphurous acid while being unchanged itself. While the emotions of the poet are essential to the process of making poetry, they are, like the platinum, not affected or included in the art that is produced.
In this, Eliot takes exception to Wordsworth's famous definition of poetry as "emotion recollected in tranquility." For Eliot, poetry is none of that: it is instead a deliberate attempt to construct new art from one's relation to the poetic tradition.