T. S. Elliot's "Impersonal Theory of Poetry" describes the poet as a chemist. The poet is a mixer, a channel or medium through which traditions, emotions and meanings flow. The poem is the finished product. Elliot rejected the idea that poetry comes from the metaphysical unity of the poet's soul; a Romantic notion. Rather, Elliot saw the poet as more like a synthesizer of feelings via language. This view of the poet is depersonalized. It is impersonal. The poet as a mere medium for emotion is unaffected by it. Elliot notes that successful poetry is impersonal because it is outlives the poet; it can become "timeless."
The objective correlative is a set of objects, events and situations which correlate with an emotion. Water, sun, time and soil are the set of objects needed for a plant to grow. Likewise, Elliot said there would need to be an object correlative for happiness, despair, etc. The objective correlative is then, a formula for an emotion. Running with the idea of the poet as a chemist, the objective correlative is a formula for describing an emotion. This is then a device that will result in impersonal poetry because the author does not have to get emotionally involved or be affected by composing the poem. He/she is just implementing a formula.
Objective correlative is a formula. Impersonal poetry is the process and the final artistic product of poetry where the poet composes timeless poetry and is emotionally distanced from that process and product. A very un-Romantic notion.