He says that a person's focus at any given time is to either approach pleasure or avoid pain.
n. a pain;a pain in the neck：讨厌的人（或事物）；讨厌的人
Why is life such a pain?
I'm sorry for being such a pain.
Beach hustlers are a pain in the neck.
n. on pain of sth/under pain of sth：违者以…处罚
All persons are commanded to keep silent, on pain of imprisonment.
How well I understood the confusion and pain of her parents.
Her grey eyes seemed filled with pain.
She was suffering from back pain.
She complained of severe pains in her chest.
To help ease the pain, heat can be applied to the area.
I felt a sharp pain in my lower back.
She phoned the doctor because she had a pain in her chest.
She's in a lot of pain.
She was writhing on the floor in pain, holding her stomach.
He's a pain, that man.
The car isn't mended yet, which is a pain.
She can be a real pain sometimes.
Getting up so early is a right pain.
What a pain moving house is!
This public acknowledgment of Ted's disability pained my mother.
n. a pain in the arse;a pain in the ass：烦心的事；讨厌的人