The Good in Man by Baruch Spinoza

Since we have shown in the preceding chapter how the Passions arise from the error of Opinion, let us now see here the effects of the two other modes of Knowing. And first of all, [the effect] of what we have called True Belief.

This shows us, indeed, what a thing ought to be, but not what it really is. And this is the reason why it can never unite us with the object of our belief. I say, then, that it only teaches us what the thing ought to be, and not what it is; between these two there is a great difference. For, as we remarked with reference to the example taken from the Rule of Three, when anyone can, by the aid of proportion, find a fourth number that shall be related to the third as the second is to the first, then (having used division and multiplication) he can say that the four numbers must be proportional; and although that is so, he speaks of it none the less as of a thing that is beyond him. But when he comes to see the proportion in the way which we have shown in the fourth example, then he says with truth that the thing is so, because then it is in him and not beyond him. Let this suffice as regards the first [effect].