The Categorical Imperative
Categorical imperative was formulated by the western philosopher Immanuel Kant. His philosophy was deontological i.e. based on duty.
What is our duty then?
Our duty is to act in such manner that we would want everyone else to act in a similar manner in similar circumstances.
Immanuel Kant:
He was born in 1724 in the East Prussian town of Konigsberg, the son of a Master Saddler. He lived there particularly all his life until he died at the age of eighty. He influenced eighteenth century philosophy more than any other western thinker. His writings established a permanent contribution to epistemology (the branch of philosophy that study the nature of knowledge; its foundations, and its extent and validity) and ethics. Groundwork of the Metaphysic of Morals (1785) and Critique of Practical Reason (1788) are his important books in ethics.
Categorical Imperative:
Kant formulates the moral law as categorical imperative. By this he means that the moral law is "categorical or that it applies to all the situations. It is, moreover, "imperative" which means it is commanding and therefore absolutely authoritative. "Categorical" here means unconditional, without any exceptions. Right is right and must be done even under the most extreme conditions. What is morally right we ought to do even if the sky should fall, that is despite whatever consequences may follow.
Categorical imperative is a theory of moral obligation. It is intringically valid. It is a theory of duty. It says "your duty should be good intentioned."First reason is used to make a law and if once the law is set, you must follow it on any circumstances" -this is categorical imperative. Here, the rule set must not only be universal to others only but it should be universal to oneself. Categorical imperative is contrast with hypothetical imperative.

Hypothetical Imperative:
It says "ends are based on physical needs." You have to do actions that are determined by ends. Here ends are physical needs. You do something only to get something.
Four principles of the principle (categorical imperative):
Kant formulates this 'categorical imperative' in the following four ways:
1. The formula of the Universal law of nature: Your act should become universal law of nature. Act as you would wish all other rational people to follow; as if it were a universal law.
2. The formula of the End itself (Humanity formula): Treat humanity as an end. Your action should be guided by humanity. We must not exploit other people to our advantage. We should not do so because every man is an end in him/herself. But this does not only apply to others, it also applies to yourself. You also must not exploit yourself as a mere means to achieving something either.
3. The formula of Autonomy: Act so that through your maxims you could be a legislator of universal law. You should be able to give law to the world. Your act should be able to become law.
4. The formula of Kingdom of Ends: You act according to what the members who make universal law do. Do as the trendsetters have done. There is kingdom of final results, you follow that.
Either you become a pathfinder and establish a rule; if you can't, you follow those rules set by others because to establish/set those rules reason has been used.
Kant's basis for ethics:
Kant said that ethics should be based on human reason, not in faith on God. So, he cancelled God. He said that we use reason to establish a rule and if once rules are formulated, you shouldn't use reason again. You should just follow those rules. Kant has also defined two types of duties and they are:
i. Perfect duty: Duty obliged to do all the time.
ii. Imperfect duty: Duty that should be done as often as possible.