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.
ACTIVITY.
An ACTIVE life increases not only the powers of the body, but also those of the mind; while indolence is the destruction of both. If a man love his neighbour in a certain degree, and take the first opportunity of putting that love into ACTION, he will then love his neighbour better than he did before, or in a higher degree; and will therefore be more ready to serve him on a future occasion, than if he had omitted the first ACT of benevolence. This is an invariable truth, provided the ACT proceed from disinterested motives; the reason of which is grounded in this immutable law, that all influx is proportioned to efflux; or in other words, That in proportion as man puts forth himself into ACTUAL uses, in the same proportion the life which flows into him from the Lord, becomes fixed within him, and forms a plane for the reception of more life. A life of ACTIVITY, therefore, when under the direction of genuine wisdom, enlarges every faculty of the human soul, and at the same time capacitates man for the most noble and exquisite enjoyments.