Belief I define to be the healthy act of a man's mind. It is a mysterious indescribable process, that of getting to believe;—indescribable, as all vital acts are.
 Thomas Carlyle, On Heroes, Hero-Worship, and The Heroic in History (1841). copy citation

Context

“The man, I say, is become spiritually a paralytic man; this godlike Universe a dead mechanical steam-engine, all working by motives, checks, balances, and I know not what; wherein, as in the detestable belly of some Phalaris'-Bull of his own contriving, he the poor Phalaris sits miserably dying!
Belief I define to be the healthy act of a man's mind. It is a mysterious indescribable process, that of getting to believe;—indescribable, as all vital acts are. We have our mind given us, not that it may cavil and argue, but that it may see into something, give us clear belief and understanding about something, whereon we are then to proceed to act. Doubt, truly, is not itself a crime.” source