When you're in pain, it's hard to keep your interest in other things. You feel that nothing's more important than finding some way to stop the pain. That's why pain is so powerful: it makes it hard to think of anything else. Pain simplifies your point of view.
When something gives you pleasure, then, too, it's hard to think of other things. You feel that nothing's more important than finding a way to make that pleasure last. That's why pleasure is so powerful. It also simplifies your point of view.
Pain's power to distract us from our other goals is not an accident; that's how it helps us to survive. Our bodies are endowed with special nerves that detect impending injuries, and the signals from these nerves for pain make us react in special ways. Somehow, they disrupt our concerns with long-term goals — thus forcing us to focus on immediate problems, perhaps by transferring control to our lowest-level agencies. Of course, this can do more harm than good, especially when, in order to remove the source of pain, one has to make a complex plan. Unfortunately, pain interferes with making plans by undermining interest in anything that's not immediate. Too much suffering diminishes us by restricting the complexities that constitute our very selves. It must be the same for pleasure as well.
We think of pleasure and pain as opposites, since pleasure makes us draw its object near while pain impels us to reject its object. We also think of them as similar, since both make rival goals seem small by turning us from other interests. They both distract. Why do we find such similarities between antagonistic things? Sometimes two seeming opposites are merely two extremes along a single scale, or one of them is nothing but the absence of the other — as in the case of sound and silence, light and darkness, interest and unconcern. But what of opposites that are genuinely different, like pain and pleasure, fear and courage, hate and love?
In order to appear opposed, two things must serve related goals — or otherwise engage the selfsame agencies.
Thus, affection and abhorrence both involve our attitudes toward relationships; and pleasure and pain both engage constraints that simplify our mental scenes. The same goes for courage and cowardice: each does best by knowing both. When on attack, you have to press against whatever weakness you can find in your opponent's strategy. When on defense, it's much the same: you still must guess the other's plan.