Mechanistic view - Biological basis:

While a person's overall happiness is not objectively measurable this does not mean it does not have a real physical component. The neurotransmitter dopamine, operating along the mesolimbic pathway and upon the nucleus accumbens, is involved in desire and seems related to pleasure. Pleasure can be induced artificially with drugs, most directly with opiates such as morphine and heroin, which block dopamine inhibitors.

Nevertheless, the exact chemicals and processes which correlate with happiness do not define the concept of happiness, they simply describe its biological "implementation". We might speculate that other implementations are possible, even if they have yet to be observed.

It is possible, however, to describe possible functions of happiness in biological terms. One such attempt is referred to as Darwinian happiness. Darwinian happiness is based on the fact that animals are equipped with the propensity for both positive and negative feelings and sensations. By understanding the underlying evolutionary background for how these sensations arise, one may gain insight in how to ensure that the neurological processes that add to a positive mood will tend to dominate.

Difficulties in defining internal experiences:

It is probably impossible to objectively define happiness as we know and understand it, as internal experiences are subjective by nature. It is almost as pointless as trying to define the color green such that a completely color blind person could understand the experience of seeing green. While we can not objectively express the difference between greenness and redness, we can certainly explain which physical phenomena cause green to be observed, and can explain the capacities of the human visual system to distinguish between light of different wavelengths, and so on. Likewise, in the following sections, we will not attempt to describe the internal sensation of happiness, but will instead concentrate on defining its logical basis. Importantly, we will try to avoid circular definitions -- for instance, defining happiness as "a good feeling", while "good" is defined as being "something which causes happiness".

In non-human animals:

For non-human animals, happiness might be best described as the process of reinforcement, as part of the organism's motivational system. The organism has achieved one or more of its goals (pursuit of food, water, sex, shelter, etc.), and its brain is in the process of teaching itself to repeat the sort of actions that led to success. By reinforcing successful decision paths, it produces an equilibrium state not unlike positive-to-negative magnets. The specific goals are typically things that enable the organism to survive and reproduce.

By this definition, only animals with some capacity to learn should be able to experience happiness. However, at its most basic level the learning might be extremely simple and short term, such as the nearly reflexive feedback loop of scratching an itch (followed by pleasure, followed by scratching more, and so on) which can occur with almost no conscious thought.

In humans:

When speaking of animals with the ability to reason (generally considered the exclusive domain of humans), goals are no longer limited to short term satisfaction of basic drives. Nevertheless, there remains a strong relationship of happiness to goal fulfillment and the brain's reinforcement mechanism, even if the goals themselves may be more complex and/or cerebral, longer term, and less selfish than a lower animal's goals might be.

Philosophers observe that short-term gratification, while briefly generating happiness, often requires a trade-off with negative repercussions in the long run. Examples of this could be said to include developing technology and equipment that makes life easier but over time ends up harming the environment, causing illness or wasting financial or other resources. Various branches of philosophy, as well as some religious movements, suggest that "true" happiness only exists if it has no long-term detrimental effects. Utilitarianism is a theory of ethics based on quantitative maximization of happiness.

From the observation that fish must become happy by swimming, and birds must become happy by flying, Aristotle points to the unique abilities of man as the route to happiness. "Of all the animals only man can sit and contemplate reality. Of all the animals only man can develop social relations to the political level. Thus the contemplative life of a monk or professor, or the political life of a military commander or politician will be the happiest."