As knowledge is no more arbitrary than perception; so, I think, assent is no more in our power than knowledge.
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“But that a man should afford his assent to that side on which the less probability appears to him, seems to me utterly impracticable, and as impossible as it is to believe the same thing probable and improbable at the same time.
16. Where it is in our Power to suspend our Judgment.
As knowledge is no more arbitrary than perception; so, I think, assent is no more in our power than knowledge. When the agreement of any two ideas appears to our minds, whether immediately or by the assistance of reason, I can no more refuse to perceive, no more avoid knowing it, than I can avoid seeing those objects which I turn my eyes to, and look on in daylight; and what upon full examination I find the most probable, I cannot deny my assent to.” source