The Pleasure Principle is a Driving Tendency of the Physical Unit. The Pleasure Principle, first identified by Sigmund Freud), is the instinctual seeking of pleasure and avoiding of pain in order to satisfy biological and psychological needs. The Pleasure Principle is the primary energetic motor for the Physical Unit generally, and the Bodily Ego specifically.
The Pleasure Principle is a powerful Driving Tendency designed and implemented to ensure the Physical Unit always seeks conditions conducive to Development. Other aspects of the Physical Unit were designed in support of this drive. Defense Mechanisms have evolved, and are invoked in situations of pain, to help the Physical Unit avoid and escape emotional, psychological, physical, and spiritual pain.
Note that salutatory Driving Tendencies may be subverted by Conditioning and/or Ideology. Unless the Physical Unit is under severe threat or attack, the Bodily Ego will typically use thought to guide action. The Pleasure Principle may be thus circumvented by ideological statements, like "what doesn't kill you makes you stronger," that suggest pain is salutatory. In situations where ideology has corrupted basic common sense, damage to the physical unit may occur, damage may accumulate, and Pathology may be the short, and certainly will be the long term, result.