An unexpected turn of fancy. Johnson quotes for the meaning 'sentiment as distinguished from imagery,' Pope's lines
'Some to conceit alone their works confine
And glitt'ring thoughts struck out at ev'ry line.' Essay on Criticism, 289.
The conceits here complained of were fantastical expressions of comparisons between things as unlike as possible. For the 'example of Cowley' cf. Johnson's life of that poet, where he says:—
'About the beginning of the seventeenth century appeared a race of writers that may be termed the metaphysical poets.....The metaphysical poets were men of learning, and to shew their learning was their whole endeavour : but unluckily resolving to shew it in rhyme, instead of writing poetry they only wrote verses. . . . .Their thoughts are often new but seldom natural ; they are not obvious, but neither are they just ; and the reader, far from wondering how he missed them, wonders more frequently by what perverseness of ingenuity they were ever found..... The most heterogeneous ideas are yoked by violence together, nature and art are ransacked for illustrations, comparisons, and allusions ; their learning instructs and their subtlety surprizes; but the reader commonly thinks his improvement dearly bought, and though he sometimes admires, is seldom pleased. . . . . Milton tried the metaphysical style only in his lines on Hobson the Carrier; Cowley adopted it and excelled his predecessors, having as much sentiment and more music.'
As an example, among many others, Johnson himself quotes Cowley :—
' All armed in brass, the richest dress of war
(A dismal glorious sight!) he shone afar.
The sun himself started with sudden fright
To see his beams returned so dismal bright.'
Cowley, says Johnson, was almost the last of that race, and undoubtedly the best.