Sigmund Freud wrote about two principles that, to some extent, govern how we act. The pleasure principle drives us toward pleasure and away from pain, without any real regard for the circumstance; think of it as a desire for instant gratification. On the other hand, the reality principle is also at its heart a pleasure-seeking impulse, but one that is willing to defer whatever pleasure is being pursued to account for whatever slings and arrows reality is throwing at the pleasure-seeker at any given time.
More or less, the difference between the two is the difference between eating a pint of ice cream all at once because you feel like it, and eating some and saving the rest for later because you know you'd regret having blown your diet if you went with the former option. Generally speaking, the pleasure principle is what children and other immature egos abide by, and the reality principle is its grown-up equivalent. Both boil down to the fact that we're all pleasure-seeking individuals at the core, whether we've made provisions for real life or not.
But a couple of examples that Freud found threw a hell of a wrench into that theory.
Fort/Da
Freud's grandson, aged one and a half, had a favourite toy—a wooden spool attached to a piece of string—and a favourite game involving it: he would throw it over the edge of his cot, exclaiming "Fort", then pull it back towards himself, exclaiming "Da". The child would do the same thing with other small toys, saying "Fort" when he threw them away and "Da" when they were brought back.
Freud, unsurprisingly, concluded that the game had symbolic meaning: the spool was thought to represent the boy's mother, and by throwing it away and bringing it back, the boy was learning how to manage the anxiety associated with her absence. The game also represented the boy's relationship with his mother: even when he couldn't see the wooden spool after having pitched it over the side of his cot, the boy knew that it was still there and could be brought back. His mother was the same way—even when she wasn't in the room, he could still recall her having been there, and assume that she would return.
Symbolism is well and good, but how repeatedly throwing away his favourite toy fits in with the pleasure principle is a bit less clear—certainly bringing back the spool after having "lost" it over the edge of the cot is pleasurable, but there's nothing instant-gratification about throwing it away again once he has it back.
Another problem was posed by the experiences of soldiers who fought in the First World War. If you talk to someone suffering from post-traumatic stress disorder, as Freud talked to soldiers, you might notice something strange: even if he wants nothing more than to be able to stop thinking about the trauma he underwent, he probably experiences involuntary flashbacks to it. He wants to forget, but his head won't let him, and he relives it over and over again in dreams or, even more miserably, waking nightmares.
This violates the pleasure principle even more obviously than the fort/da game. At first, Freud thought that flashbacks, too, had symbolic significance: maybe, like the child's need to manage his anxiety about his mother, flashbacks were a means of mastering the trauma after the fact so that the traumatised person could cope more effectively with it.
But that idea was discarded, and eventually Freud settled on a different theory that would encompass both experiences: that there is a deeper principle underlying the pleasure principle and reality principle. This underlying principle, described in Beyond the Pleasure Principle, is the death drive (a.k.a. death instinct), and it describes every individual's need to die.
"The purpose of all life is death"
The death drive is sometimes conflated with Thanatos (opposed to Eros), though Freud never used that term himself. It is similar to destrudo, the destructive impulse, but the two mustn't be confused; the death drive doesn't seek the death or destruction of other individuals, only one's own death; nor is it aggressive, as destrudo implies.
When Freud says of the life and the death drive that "the purpose of all life is death", he is not referring to a conscious wish or an imperative that we generally are aware of, but rather an unconscious desire that is predicated on the principle that "all the organic instincts are conservative […] and tend toward the restoration of an earlier state of things" (Freud, 613). The death drive is an expression of this fundamentally conservative nature of living things: they seek to preserve a state of homeostasis, where they are not subject to excitations that might cause pain. (This is the pleasure principle, again.)
In very literal terms, a living thing's conservative tendencies are based on chronology, as "inanimate things existed before living ones" (613). Therefore, the earliest possible state to which a living thing might regress is that of the inanimate—so the organic strives to become inorganic; or, in other words, the living seeks to become dead. It is only in the inorganic state that an organism can be wholly free from the threats posed by stimuli (excitations), and completely secure in the 'ground state', or homeostasis, in which it is most comfortable.
The idea that all living things are seeking to die at first appears contrary to their apparent tendencies toward self-preservation, which seem to be instinctive. Freud circumvents this potential problem by postulating "that instinctual life as a whole serves to bring about death" (614). This means that every organism has an immanent end—a natural telos toward which it is progressing during its lifetime. Consequently, it does seek to avoid an untimely death; this is the 'self-preservation instinct' that we observe in living things (as well as in ourselves). Furthermore, the death drive isn't a 'proper' instinct—it is a "component instinct" that exists not to prevent death altogether but rather "to assure that the organism shall follow its own path to death" (614).
What this means for the pleasure principle is that we're still pleasure-seeking individuals, but we're finite individuals, too; and our understanding of that finitude goes deeper than any pleasure—instant or deferred—ever could.
Page citations come from a stupid course reader with a fragment of Beyond the Pleasure Principle that had its own pagination, but no publishing information provided. To hell with that. The rest is gleaned from my own reading and a few hours of Freud-related lectures from a class at King's entitled "Memento Mori: Reflections on Death", which was, in fact, quite a bit less dreary than its name might imply.