Here's another interesting doctrine from Buddhism for ya:
The fundamental 'three marks' of all conditioned phenomena are that they are impermanent (anicca), dukkha, and not-self (anatta). Buddhism emphasises that change and impermanence are fundamental features of everything, bar Nibbāna. Mountains wear down, material goods wear out, and all beings, even gods, die. The gross form of the body changes relatively slowly, but the matter which composes it is replaced as one eats, excretes, and sheds skin cells. As regards the mind, character patterns may be relatively persistent, but feelings, moods, ideas, etc. can be observed to constantly change.
It is because of the fact that things are impermanent that they are also dukkha: potentially painful and frustrating. Because they are impermanent and unsatisfactory, moreover, they are to be seen as not-self: not a permanent, self-secure, happy, independent self or I. They are 'empty' (suñña) of such a self, or anything pertaining to such a self.
In the Buddha's day, the spiritual quest was largely seen as the search for identifying and liberating a person's true self (atta). Such an entity was postulated as a person's permanent inner nature, the source of true happiness and the autonomous 'inner controller' of action. The Buddha argued that anything subject to change, anything not autonomous and totally controllable by its own wishes, anything subject to the disharmony of suffering, could not be such a perfect true self. Moreover, to take anything which was not such a self as if it were one, is to lay the basis for much suffering. This arises when what one fondly takes as one's permanent, essential self changes in undesired ways.