Discourses, That We Ought not to Be Angry with Men; and What Are the Small and the Great Things Among Men

Epictetus — The Slave

Why do you agree with something? Because it seems true to you. You can't agree with something that seems false. Why not? Because that's how your mind works. It naturally leans toward what seems true. It rejects what seems false. When things are unclear, it holds back from deciding. Want proof? Try to convince yourself it's nighttime right now. You can't do it. Try to stop believing it's daytime. You can't do that either. Try to convince yourself that the stars are an even number. It's impossible.

Epictetus — The Slave Original

What is the cause of assenting to anything? The fact that it appears to be true. It is not possible then to assent to that which appears not to be true. Why? Because this is the nature of the understanding, to incline to the true, to be dissatisfied with the false, and in matters uncertain to withhold assent. What is the proof of this? Imagine (persuade yourself), if you can, that it is now night. It is not possible. Take away your persuasion that it is day. It is not possible. Persuade yourself or take away your persuasion that the stars are even in number. It is impossible.