When any person treats you ill or speaks ill of ,you, remember that he does this or says this because he thinks that it is his duty. It is not possible, then, for him to follow that which seems right to you, but that which seems right to himself. Accordingly, if he is wrong in his opinion, he is the person who is hurt, for he is the person who has been deceived; for if a man shall suppose the true conjunction to be false, it is not the conjunction which is hindered, but the man who has been deceived about it. If you proceed, then, from these opinions, you will be mild in temper to him who reviles you: for say on each occasion, It seemed so to him.
This reminds me of this quote from Adam Smith's Theory of Moral Sentiments:
Every faculty in one man is the measure by which he judges of the like faculty in another. I judge of your sight by my sight, of your ear by my ear, of your reason by my reason, of your resentment by my resentment, of your love by my love. I neither have, nor can have, any other way of judging about them.
The Smith quote acts as a precursor to Epictetus'; it lays the foundation for why we make the mistake of assuming that people follow what seems right to us. But as Epictetus points out, even if we cannot understand it, people act according to their own judgements; and if they are wrong, it is no business of ours. We should be generous in our judgements of others' opinions, a theme that is repeated in part 45.