How can one know nature:
"Nature is organized through cognitive forms (categories) of reason." A person as a natural being obeys these categories of reason, but not only because there is will. One is also a free being because of the will (Kant calls it a "moral imperative"). The two worlds - nature and the moral imperative interact with each other. Freedom of will and the innumerable nature of man are the basis of his behavior.
Practical philosophy (Kant treats this concept of ethics and law) is based on a priori ideas that can not be interpreted as the exact sciences. The laws of morality can not be deduced from the available, from the existing relationships between people, but from reason (from the ideas of reason, as reason is perceived as a higher "floor" of the mind).
Kant thinks that ethics should be built without human behavior taking into account particular circumstances. Ethics should be universal, universal. One has to act with respect to others as he would like to act on him (the categorical imperative of Kant).
It contains only the general idea of duty of mankind, not a prescribed prescription of behavior which gives some freedom of behavior but within the prescription that man is part of humanity and has a certain duty to him.Man's behavior must be governed by the fact that the other person is like him and not to use it as a means of personal use.Kant's idea of eternal peace (about the existence of different peoples). The Problem of War and Peace ("Towards Eternal Peace"). Kant creates a theory of world or eternal peace in which peace can be achieved by law (legal) regulation of states.
