In the practice of art, as well as in morals, it is necessary to keep a watchful and jealous eye over ourselves; idleness, assuming the specious disguise of industry, will lull to sleep all suspicion of our want of an active exertion of strength. A provision of endless apparatus, a bustle of infinite enquiry and research, or even the mere mechanical labour of copying, may be employed, to evade and shuffle off real labour, — the real labour of thinking.

Joshua Reynolds (1723-1792) British painter, critic

Speech to the Royal Academy, London (10 Dec 1784)
    (Source)

Paraphrased over a long period of time (and still attributed to Reynolds) as: "There is no expedient to which a man will not resort to avoid the real labor of thinking."

The lecture was later described as the Twelfth Discourse in a 1797 collection of Reynolds' works.

Often attributed to Thomas Edison. More information here.