Optimism, the opposite of pessimism, is a lifeview where one looks upon the world as a positive place. Optimists generally believe that people are inherently good. They have a "positive" outlook on life, believing that given time, things will work out in the end. A common example used to illustrate optimism is this question: given a glass that has been filled halfway, is it half filled, or half empty?

In philosophy, optimism is linked with the name of Gottfried Leibniz, who held that we live in the "best of all possible worlds," a theodicy for which he was famously mocked by Voltaire in his satirical novel Candide. Its opposite is philosophical pessimism. Perhaps even more optimistic than Leibniz was the anarchist philosopher William Godwin. He hoped that society would eventually reach the state where all violence and force would be replaced by calm reason, that matter could eventually be made subservient to mind and that the secret of immortality could be discovered. Some are surprised to learn that a freedom-loving anarchist like William Godwin disapproved of suicide, but this was due to his optimistic belief that suicide was almost always a mistake.

Overoptimism, or strong optimism, is the overarching mental state wherein one believes things are more likely to go good for them than bad. Compare it with the valence effect of prediction, a tendency for people to overestimate the likelihood of good things happening rather than bad things.

Optimism is strongly correlated with Self-Esteem, with our psychological well-being and with our health. Seligman is the most important researcher about this object.

Energy is a fundamental quantity that every physical system possesses; it allows us to predict how much work the system could be made to do, or how much heat it can exchange. In the past, energy was discussed in terms of easily observable effects it has on the properties of objects or changes in state of various systems. Basically, if something changes, some sort of energy was involved in that change. As it was realized that energy could be stored in objects, the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms; examples are the electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a hot water heater, or the kinetic energy of a moving train. To simply say energy is "change or the potential for change", however, misses many important examples of energy as it exists in the physical world.

Energy can be used not only to produce observable change, it also is used to prevent change in which case unaided observation of this kind of energy can be difficult. For example, looking at a statue holding a 50 pound weight, the presence of energy needed to do so may not be observable. However, if you are holding up the fifty pound weight instead of the statue the need for energy to accomplish this becomes apparent. You can feel the gravitational force on you both when you are moving the weight up and when you are not moving it.

Energy can be readily transformed from one form into another; for instance, using a battery to power an electrical heater converts electrical energy into thermal energy. In the previous example of holding the fifty pound weight, the work you perform to raise the weight is observed as kinetic energy of motion which is converted to potential energy. Letting go of the weight once again transforms this stored potential energy back into kinetic energy as the weight falls under the force of gravity. The law of conservation of energy states that the total amount of energy, corresponding to the sum of a system's constituent energy components, remains constant. Scientists have also defined several forms of energy that are not easily measured by the unaided observer.