Instincts are autonomic responses to external stimuli or various environmental conditions. These innate features are both physically and psychologically built into the human organism and are the driving force for many involuntary and voluntary actions, which humans rationalize and then engage in willingly.
As described in the psychological review, "A Functional Interpretation of Human Instincts," by J.R. Kantor, found on brocku.ca, some of the very basic human instincts are: acquisition of food, expelling bodily waste, responses and desires of a sexual nature, acts of emotional expression and natural defensive reactions to stimuli adverse to survival.
Although some instinctual behavior is involuntary and takes no rationalization, such as breathing or blinking, there are many learned behaviors and recognition of past patterns that do require rationalization capabilities. However, base urges are still the driving force behind the behavior.
A wide array of human behavior, even if rationalized and deemed to be the appropriate action to take due to learned experiences and memory, are based around an instinctual nature. There are many examples of instinct-rooted action being carried out by humans every day. For instance, when someone is hungry, that person reasons that food is stored in his kitchen cabinet, utilizing reasoning capabilities driven by the base instinct of hunger.
Sexual conquest is another example of rational behavior based in instinctual desire. A person feeling the desire for a mate will use past experience, understanding of societal trends or past failures to rationalize what the best approach would be to complete his objective and find a sexual partner to satisfy his base instinct.