William James (1842 - 1910) was a psychologist and philosopher, and was recognized for writing the Principles of Psychology, which is considered to be a monumental work in the history of psychology.
James is known for the James-Lange Theory of Emotion, which he formulated independently of Carl Lange. According to the theory, an emotion is simply the mind's interpretation of certain physiological processes that occur as a response to certain stimuli.
One of James' most famous examples is that when we see a bear, we do not run because we are afraid. According to James, we see a bear and then we run, and that is why we are afraid. His explanation is that when exposed to a stimulus such as a bear, our nervous system reacts with an increased heart rate, a rush of adrenaline, or muscle tension, and our perception of those changes is what is referred to as emotion.