Kant used the idea of imperatives to express the two different types of "must dos" in life. The first is called a "categorical imperative...which [is] represented [as] an action objectively necessarily of itself, without reference to another [goal]". The second is called the hypothetical imperative which are actions "good merely as a means to something else".
To make this clearer consider a starship on an 100 year mission. Physical requirements (like run x number of laps in y time) are ok to impose on the crew because one does not have to go on this mission to have a happy life; physical requirement belong to the hypothetical imperative. But imposing thought requirements and belief requirements would violate the categorical imperative.
For example, the captain of the starship and the design team must not place any limitation or requirements on the number of children crew members must or can have because that asks the crew to conform with a certain thoughts and that can make them miserable. It is different from physical requirements because it can be designed around. Furthermore, it is different because if we find ourselves violating anything that must be done to execute something that could be done then we find ourselves in moral fault. This is because it is far more important, because the proof for categorical imperatives does not rely on our wishes, to preserve categorical imperatives and abandon hypothetical ones.
Yes, this means we might not be able to do the 100 year starship mission at all.
