John Locke made contributions to both pure philosophy and to political philosophy.
Locke's purely philosophy ideas dealt with empiricism, distinguishing between an object's primary and secondary qualities, a rejection of innate ideas, and his famous "tabula rasa" metaphor: humans are born like "blank pieces of paper", so that the source of all our knowledge is, and must be experience.
Politically, Locke is known for locating the source of sovereign authority in the people; the legitimacy of a government comes from the consent of the governed. He also stressed property rights, and one's duties to society.
There is a connection between Locke's purely philosophical thoughts and his political doctrines: given that human beings are born as "blank slates", then Locke would never allow the defendant in a criminal trial to excuse himself from responsibility for his crimes because "he was born that way". Locke would argue that one could not claim an in-born factor which would cause one to become a thief, murderer, arsonist, or rapist. There would be no genetic determiner of behavior, as a consequence of the denial of innate knowledge.
Locke would be likely to allow a the attorney defending the accused to claim that "environmental factors added up to make him that way": negative factors in an individual's experiences could, perhaps, drive that individual to develop "mal-adaptive coping mechanisms". These, developed subconsciously or semi-consciously or unconsciously, would be neither fully responsible moral choices nor innate/genetic determinations of behavior.
So Locke's abstract philosophical psychology could have a very practical impact in the courtroom.
