Teleology and deontology are two of the three major approaches to the study of ethics. Deontological ethics is a rule driven system, with moral status contingent on adherence to rules. Teleology, frequently called consequentialism, bases morality on the end result of an action. Though there are many differences between these two ethical approaches the most significant is that deontology studies actions and rules while teleology studies consequences.
The major difference between these two approaches lie in the topic of focus. Deontological ethicists focus on actions and rules. Deontological ethical systems have at their centre a set of rules. These rules may differ from system to system. For example, the Divine Command Theory states that an action is right if God has declared it to be right. In this instance, the rules are set by God or another relevant deity and only actions that comply with those commands are moral. Note that in this theory no weight is given to intention, desires or consequences of the actions in question.
The deontological approach does not rely exclusively on a religious foundation. Other deontological systems are founded on the non-aggression principle which states that there is no moral justification for the initiation of force against another human being. This principle precludes physical violence as well as political coercion such as taxation or abridgment of speech. Deontological libertarians such as Murray Rothbard championed this concept.
Teleology does not focus on actions themselves or how closely they adhere to a system of rules. Teleological ethics, which is mostly referred to as consequentialism, is concerned with the end effect. The essence of all forms of teleological ethics is best stated by the founder of Utilitarianism, Jeremy Bentham: "the greatest good for the greatest number." Accordingly, the impact of society as a whole is what determines morality. This approach differs from deontology in that there is no set of hard and fast rules in place. Actions may be moral or immoral depending on circumstance.
Modern deontology has its roots in the Enlightenment writers of continental Europe. Rene Descartes of France was the first Enlightenment philosopher and his ideas influenced subsequent writers in all scientific and philosophical disciplines, including ethics. The most influential deontological writer, however, was Immanuel Kant of Konigsberg, Prussia (present day Kaliningrad, Russia).
On the other hand, teleological ethics, mainly in the form of Utilitarianism, evolved in England throughout the 18th and 19th centuries. Jeremy Bentham founded the school of Utilitarianism but it was John Stuart Mill who later refined the theory in the 1800s.
Deontology in the form of Divine Command Theory has existed since the times of Ancient Greece when the concept was addressed in Plato's "Euthyphro." The debate continued with medieval scholars such as Thomas Aquinas up until the Renaissance and finally gave way to Kantian ethics in the 1700s. Deontology's long life is contrasted with teleological ethics, which was not developed into a formal ethical theory until the 18th and 19th centuries.