20 In the case of those who profess to take reason as their sole guide, there would hardly be found, if, indeed, there ever could be found, unity of doctrine. Indeed, the art of knowing things as they really are is exceedingly difficult; moreover, the mind of man is by nature feeble and drawn this way and that by a variety of opinions, and not seldom led astray by impressions coming from without; and, furthermore, the influence of the passions oftentimes takes away, or certainly at least diminishes, the capacity for grasping the truth. On this account, in controlling State affairs means are often used to keep those together by force who cannot agree in their way of thinking.
Source: Sapientiae Christianae (Vatican.va)