T.S. Eliot's "The Metaphysical Poets" was first published as a review of J.C. Grierson's Metaphysical Lyrics and Poems of the 17th Century. T.S. Eliot did far more than just review Grierson's book, however. He also raised new questions and new ways of thinking about Metaphysical poets like John Donne, and he introduced new terms for his ideas that are still used in literary criticism today.
Eliot begins by praising Grierson's book, but he points out that the label "Metaphysical" hasn't been a useful way of thinking about seventeenth-century poets who produced similar work. For example, "metaphysical" had often been used to mock or criticize these works. Even when used seriously, the term didn't help literary scholars determine whether the "metaphysical" poets had developed a new poetic movement, or were just continuing an older tradition.
The first problem, of course, is how to define "metaphysical" poetry. Eliot groups prominent 17th century poets into three schools: the late Elizabethan, including Donne, Marvell and Bishop King; the courtly, including Jonson and Prior; and the devotional, including Herbert, Vaughan and Crashaw. Eliot notes that it's hard to find characteristics that are common to all these poets and that could be used to define them all as "metaphysical."
Eliot tries to find these common characteristics, however. He starts by examining examples of these poets' work for characteristics that are often called "metaphysical." But these characteristics are also found in Elizabethan literature, suggesting that the "metaphysical" poets borrowed them from the previous generation rather than inventing them.
Next, Eliot analyzes the "metaphysical" poets by using Dr. Johnson's definition of metaphysical poetry: it's poetry in which "the most heterogeneous ideas are yoked violence together"; in other words, totally different ideas are made to work together in the poem. Eliot points out that this definition could apply to all poetry, so it's basically useless for defining "metaphysical" poetry specifically.
If we can't define "metaphysical" poetry by its faults, Eliot decides, he'll try defining it by its virtues. He identifies two: Unification of Sensibility and Dissociation of Sensibility. These two terms are still used in literary criticism today.
Eliot's "Unification of Sensibility" refers to the fusion and recreation of thought and feeling within the poem, even when these differ from or even contradict one another. To Eliot, this unification of sensibility arises from being well-read, thinking about what one has read, and allowing that reading and thought to affect feeling. A poem displaying "unification of sensibility" is one that shows both the thought process and the way it modifies feelings.
Eliot uses "Dissociation of Sensibility" to describe what happens in English poetry after the metaphysical poets. Dissociation of sensibility occurs when poets can either write intellectual "thinking" poems or emotional "feeling" poems, but not poems that do both. For instance, Eliot considers poets like John Milton to be "thinking" and poets like William Wordsworth to be "feeling."
Eliot believes that both the "thinking" and "feeling" poets did make important contributions to English poetry, just that neither group managed to unite thought and feeling the way the "metaphysical" poets did. For example, he credits Milton and Dryden with refining the English language, but at the expense of more crude and childish portrayals of emotion in poetry. Meanwhile, poets like Shelley and Keats produced more vivid imagery by meditating on feeling, but they struggled to convey both the image and the feeling together.
Although Eliot doesn't use the terms "Restoration" or "Romantic" poets, his division between "thinking" poets and "feeling" poets more or less tracks how we treat these eras of English literature today. We tend to see Restoration poetry as more thought-based, while the Romantic poets expressly stated that their goal was to capture "the spontaneous overflow of powerful feelings" in poems, as Wordsworth put it.
Thus, Eliot defines the "metaphysical" poets as the poets who had a unified sensibility, which allowed them to fuse together thought and feeling into a new poetic experience that the poets who followed them did not or could not imitate. Eliot sees the "metaphysical" poets as fully-developed poets who advanced English poetry with their work.