To understand fear as an emotion, it is necessary to understand something about living tissue.

Living tissue exists because it has the ability to "know" what is harmful and what is beneficial. This is true even in primitive single-cell organisms. Not only does living tissue know how to make these distinctions, it also has an ability to protect itself from noxious influences. This protection may be as simple as becoming impervious to electrochemical energy. It also often results in movement to escape from threat.

Psychologically, this reaction to threat is identified as anxiety. More commonly people refer to it as the emotion, fear. In general usage, the terms anxiety and fear are used interchangeably.

Little is known about the possible existence of emotions in primitive animal forms. However, it is well-known that mammals share an emotional response to threat. That response is commonly referred to as stress - or more properly as the stress response.

There are two other emotions that are derivatives of anxiety. One is anger. The other is sadness. While both anger and sadness seem to occur almost instantly, each is preceeded by an instant of anxiety.