Sometimes people try to defend evils by drawing a false analogy with the natural sciences.
“Lots of things in science are counter-intuitive,” they may say. “For example, you might think the sun is only a little higher than the clouds, but in reality it’s so far from us that its light takes more than eight minutes to get here. Well, lots of things in ethics are counter-intuitive too. You might think no one should ever have an abortion, but in reality it’s often a necessary evil.”
The flaw in the analogy is that propositions like “Never commit evil, even for the sake of some good” aren’t like sense impressions, which can be mistaken; they are self-evident first principles of reason, like “nothing can both be and not be in the same sense at the same time.” They cannot be mistaken; in fact, they are the means by which we discover that other things are mistaken. Anyone who tries to suggest that something can both be and not be has missed the point of what being is. Anyone who supposes that evil may sometimes be done has failed to grasp what evil is.
By the way, I am not suggesting that nothing in ethics is counter-intuitive. To a person habituated to vice, virtue itself seems counter-intuitive. Something tells me that astronomy teachers don’t face that particular difficulty.