The extreme of the mechanistic view is the theory of instincts. An instinct in an innate biological force that predisposes the organism to act in a certain way. The behaviour of animals had long been attributed to instincts, since they had no soul or intellect and could not operate on the basis of reason.
When Darwin suggested that there was no sharp distinction between humans and animals, the door was opened for the use of instincts in explaining human behaviour.
The strongest advocate of instinct theory was the psychologist William McDougall who maintained that all of our thoughts and behaviour were the result of instincts. In the book Social Psychology, published in 1908, McDougall mentioned the following instincts:
McDougall thought these instincts were inherited and compelling sources of conduct, but modifiable by learning and experience. He expanded his list to 18 instincts, including some that related to specific bodily needs. By modifying and combining these instincts he attempted to explain all human behaviour.
Instinct theory is diametrically opposed to a rationalistic view of human beings. Instead of choosing goals and actions a person is at the mercy of innate forces, which determine or motivate his course of action.
Psychoanalytic theory also attributed behaviour to powerful innate forces.
Freud believed that behaviour was determined by two basic energies: The life instincts which found expression in sexual behaviour and the death instincts which underlie aggressive acts.
These instincts though unconscious were powerful motivational forces.
Both psychoanalytic theory and instinct theory were influential in turning interest away from a rationalistic conception of people toward a motivational view that saw behaviour as the result of unconscious, irrational forces within the individual.