Any acceleration or retardation of the natural run of perceptions, is painful even where it is voluntary. And it is equally painful to alter that degree of variety which nature requires. Contemplation, when the mind is long attached to one thing, soon becomes painful by restraining the free range of perception. Curiosity and the prospect of advantage from useful discoveries, may engage a man to prosecute his studies, notwithstanding the pain they give him; and a habit of close attention, formed by frequent exercise, may soften the pain. But it is deeply felt by the bulk of mankind, and produceth in them an aversion to all abstract sciences. In any profession or calling, a train of operation that is simple and reiterated without intermission, makes the operator languish, and lose his vigor. He complains neither of too great labour nor of too little action; but regrets the want of variety, and his being obliged to do the same thing over and over. Where the operation is sufficiently varied, the mind retains its vigor, and is pleased with its condition. Actions again create an uneasiness when excessive in number or variety, though in every other respect agreeable. This uneasiness is extremely remarkable, where strict attention must be given, at the same time, to a number of different things. Thus a throng of business in law, in physic, or in traffick, distresseth and distracts the mind, unless where a habit of application is acquired by long and constant exercise. The excessive variety is the distressing circumstance; and the mind suffers grievously by being kept constantly upon the stretch.