Like dogs in a wheel, birds in a cage, or squirrels in a chain, ambitious men still climb and climb, with great labor, and incessant anxiety, but never reach the top.

Sometimes ascribed to Robert Browning, this is in fact a misquotation from Robert Burton's The Anatomy of Melancholy (1621): "They [i.e. ambitious men] may not cease, but as a dog in a wheel, a bird in a cage, or a squirrel in a chain, so Budaeus compares them; they climb and climb still, with much labour, but never make an end, never at the top".