Kant starts his argument by making a distinction between a posteriori statements and a priori statements. Kant held that an a posteriori statement is one that is based on experience of the material world, whereas an a priori statement requires no such knowledge; it is known independent of the phenomenal world.
Kant then continues to make a further distinction, between analytic statements and synthetic statements. An analytic statement, he claims, is one that by its very nature is necessarily true, as the predicate is included within the definition of the subject. For example, "all squares have four sides" is an analytic statement, as the predicate, i.e. the square having four sides, is part of the definition of the subject, "square". As well as being necessarily true, an analytic statement is purely explicative, as it tells us nothing new about the subject. By contrast, a synthetic statement is one in which the predicate is not included in the definition of the subject and thus is not necessarily true. A synthetic statement also tells us something new the subject.
Prior to Kant, it was widely accepted that there were only two types of statement: a priori analytic and a posteriori synthetic. Kant accepted these two statements although believed there to be a third: a priori synthetic statements. These are statements that are known independent of experience, that may or my not be true.
Kant claimed that it is these a priori synthetic principles, which are inherent within us, that form the basis of all moral decision making.
Having established this fact, Kant now seeks to establish a universal principal under which we should always act in relation to morality.
To do this, Kant identifies two imperatives or commands under which we act: hypothetical imperatives and categorical imperatives. A hypothetical imperative is something that we do as a means to an end, for example "do x to achieve y". A categorical imperative, Kant holds, is one that is an end in itself and takes the form "do x". It is a call to duty, an action done for its own sake.
Kant then introduces the idea that we should act as lawmakers when making moral decisions; "act only according to that maxim by which you can at the same time will that it should become a universal law". What Kant means by this is that we should only perform a moral action if we are willing for that action to become a universal law for everybody to follow.
Kant then introduces the practical imperative, which in summary runs as follows: " Act in such a way that you always treat humanity, whether in your own person or in the person of any other, never simply as a means, but always at the same time as an end".
Kant believed that a person with a good will is someone who acts wholly rationally, someone who disregards their selfish inclinations and acts out of a sense of duty in accordance with the categorical imperative. He believed that any fully rational moral agent would follow this theory.
To sum up Kant's approach to ethics, the following quote is useful:
"So act as if you were through your maxims a law-making member of a kingdom of ends".