Rules for judging the beauties of painting, music, and poetry; founded on a new examination of the word thought, as applied to the fine arts.
Thoughts are, generally speaking, all ideas sufficiently distinct to be conveyed by signs. When speaking with a particular reference to the belles lettres and polite arts, we mean, by thoughts, the ideas which the artist attempts to raise by his performance, in contradistinction to the manner in which they are raised or expressed.
In works of art, thoughts are what remain of a performance, when stripped of its embellishments. Thus, a poet’s thoughts are what remains of his poems, independently of the verification and of some ideas, merely serving for its decoration and improvement.
Thoughts, therefore, are the materials proposed and applied by art to its purposes. The dress in which they appear, or the form into which they are moulded by the artist, is merely accidental; consequently, they are the first object of attention in every work of art; the spirit, the soul of a performance, which, if its thoughts are indifferent, is but of little value, and may be compared to a palace of ice, raised in the most regular form of an habitable structure, but, from the nature of its materials, totally useless.
While, therefore, you are contemplating an historical picture, try to forget that it is a picture; forget the painter, whose magic art has, by lights and shades, created bodies where there are none. Fancy to yourself that you are looking at men, and then attend to their actions. Observe whether they are interesting; whether the persons express thoughts and sentiments in their faces, attitudes, and motions; whether you may understand the language of their airs and gestures; and whether they tell you something remarkable. If you find it not worth your while to attend to the persons thus realised by your fancy, the painter has thought to little purpose.
Whilst listening to a musical performance, try to forget that you are hearing sounds of an inanimate instrument, produced only by great and habitual dexterity of lips or fingers. Fancy to yourself, that you hear a man speaking some unknown language, and observe whether his sounds express some sentiments; whether they denote tranquility or disturbance of mind, soft or violent, joyful or grievous affections; whether they express any character of the speaker; and whether the dialect be noble or mean. If you cannot discover any of these requisites, then pity the virtuoso for having left so much ingenuity destitute of thought.
In the same manner we must judge of poems, especially of the lyric kind. That ode is valuable, which, when deprived of its poetical dress, still affords pleasing thoughts or images to the mind. Its real merit may be best discovered by transposing it into simple prose, and depriving it of its poetical colouring. If nothing remains, that a man of sense and reflection would approve, the ode, with the most charming harmony, and the most splendid colouring, is but a fine dress hung round a man of straw. How greatly then are those mistaken, who consider an exuberant fancy, and a delicate ear, as sufficient qualifications for a lyric poet!
It is only, after having examined the thoughts of a performance in their unadorned state, that we can pronounce whether the attire, in which they have been dressed by art, fits, and becomes well or ill. A thought whose value and merit cannot be estimated, but from its dress, is, in effect, as futile and insignificant as a man who affects to display his merit by external pomp.
Original (English translation): A General Theory of the Polite Arts, delivered in single Articles, and digested according to the Alphabetical Order of their technical Terms. By John George Sulzer, Fellow of the Royal Academy of Sciences at Berlin.
Possible sources:
1774: The Critical review: or, Annals of literature, Volume 38, ed. Tobias George Smollett.
1774: The Monthly Miscellany, 1774.
1790: The New magazine of knowledge concerning Heaven and Hell, Vol. I. This seems the most likely direct source.