As the good is in relation to things, so is the true in relation to knowledge. Now in things it is impossible to find one that is wholly devoid of good. Wherefore it is also impossible for any knowledge to be wholly false, without some mixture of truth. Hence Bede says that “no teaching is so false that it never mingles truth with falsehood.” Hence the teaching of the demons, with which they instruct their prophets, contains some truths whereby it is rendered acceptable. For the intellect is led astray to falsehood by the semblance of truth, even as the will is seduced to evil by the semblance of goodness. Wherefore Chrysostom says: “The devil is allowed sometimes to speak true things, in order that his unwonted truthfulness may gain credit for his lie.”
Thomas Aquinas, Summa Theologica (II-II, Question 172, Article 6)
