Johnson believed that the intellect without sociable interchange or occasional enforced periods of reflection will rust into melancholy. Experience has a general tendency to become a chaotic and unregulated sequence of ideas, and ideas received in the past tend to fade unless regularly refreshed. Each person is subject to these general laws of the mind, and each person also has a disposition to decline into his particular set of vices. Each of us, however, has a sluggish but voluntary power to arouse ourselves from both our collective and our individual weakness.
And the application to Pope and Swift:
Pope was ‘fretful, and easily displeased, and allowed himself to be capriciously resentful’. That phrase ‘allowed himself’ is absolute Johnson: Pope’s failure to correct his own inclination to be fretful turns his natural disposition into a moral failing. In a similar way Swift, towards whom Johnson is generally hostile, condemned himself to eventual madness by his refusal to participate in society: ‘His asperity continually increasing, condemned him to solitude; and his resentment of solitude sharpened his asperity.’ The sentence is so damning because it makes no reference to Swift’s own agency. He failed to prevent himself from becoming an isolated curmudgeon because he allowed his passions to drive his behaviour into a loop of decline, and that way madness lay: ‘His ideas, therefore, being neither renovated by discourse, nor increased by reading, wore gradually away, and left his mind vacant to the vexations of the hour, till at last his anger was heightened into madness.’
On Johnson's criticisms of the metaphysical poets, Burrow has this to say:
He believed that a heterogeneity of elements – the method of metaphysical poetry according to Johnson – was intrinsically prone to cause corruption and impermanence in poems as it was thought to do in physical bodies. Hence modes which bring together contrasting registers – notably burlesque and mock epic – are not of permanent value: ‘Burlesque consists in a disproportion between the style and the sentiments … It, therefore, like all bodies compounded of heterogeneous parts, contains in it a principle of corruption.’
I am not so sure about this. After all, Johnson explicitly praises heterogeneity of a kind in the Preface to Shakespeare:
The end of writing is to instruct; the end of poetry is to instruct by pleasing. That the mingled drama may convey all the instruction of tragedy or comedy cannot be denied, because it includes both in its alterations of exhibition, and approaches nearer than either to the appearance of life, by shewing how great machinations and slender designs may promote or obviate one another, and the high and the low co-operate in the general system by unavoidable concatenation. [...] let it be considered likewise, that melancholy is often not pleasing, and that the disturbance of one man may be the relief of another; that different auditors have different habitudes; and that, upon the whole, all pleasure consists in variety.
A play isn't a poem but this cannot be the relevant distinction. I think the principle of corruption is a red herring; Johnson's objection to the metaphysical poets is that their metaphors do not approach "the appearance of life" and are frigidly artificial, the metaphor is one of desiccation rather than rot. And Johnson's praise of the mock-epic Rape of the Lock is, as far as I remember, unreserved. As Johnson remarks about the Metaphysicals, "if their conceits were far-fetched, they are often worth the carriage"; it is better to understand his objections to Cowley and Donne were as being of a piece with his objections to "academic" poetry more generally, for being (in Burrow's phrase) "mean, donnish and unnatural."
While looking up the Rape in the life of Pope I came upon this delightful paragraph that I had unaccountably never noticed before:
 The purpose of the Poet is, as he tells us, to laugh at "the little unguarded follies of the female sex." It is therefore without justice that Dennis charges The Rape of the Lock with the want of a moral, and for that reason sets it below The Lutrin, which exposes the pride and discord of the clergy. Perhaps neither Pope nor Boileau has made the world much better than he found it; but if they had both succeeded, it were easy to tell who would have deserved most from publick gratitude. The freaks, and humours, and spleen, and vanity of women, as they embroil families in discord and fill houses with disquiet, do more to obstruct the happiness of life in a year than the ambition of the clergy in many centuries. It has been well observed that the misery of man proceeds not from any single crush of overwhelming evil, but from small vexations continually repeated.
And I think that one aspect of Johnson's lives that Burrow doesn't stress enough is that they are full of good technical criticism. The now-well-known observation that Pope's poems have too many adjacent couplets with similar rhymes, for instance, or this remark about Dryden's use of rhyme:
The rhymes of Dryden are commonly just, and he valued himself for his readiness in finding them; but he is sometimes open to objection.
It is the common practice of our poets to end the second line with a weak or grave syllable:
Together o'er the Alps methinks we fly, Fill'd with ideas of fair Italy.Dryden sometimes puts the weak rhyme in the first:
Laugh all the powers that favour tyranny, And all the standing army of the sky.
It is absolutely true that there is a difference between the two patterns, but I have never seen anybody else point this out.
I'd like to conclude this overlong post with a bleg. My friend Calista is looking for passages in novels, poems, etc. that involve grease-smeared or food-encrusted books. The purpose, as I understand it, is to endow her frequent book-besmirchings with literary associations. I can barely think of anything; examples would be welcome.