base instinct
A subconscious urge, behavior, or intuition directed by primeval, animalistic, self-serving, and/or ignoble motivations. Greed and selfishness are unfortunately two of the base instincts ingrained in the human psyche. Though he is an upstanding citizen, when he drinks, he unfortunately succumbs to his base instincts.
gut instinct
An intuition or feeling, as opposed to an opinion based on a logical analysis. My gut instinct about Kara's new boyfriend proved correct when we discovered he had a pretty extensive criminal history.