Discipline is the ability to follow a set of rules without failing. As far as I know, there are two types of discipline. The first is external discipline, which is when you follow what another person says without question. This type of discipline is useless and immoral, as it denies people their basic freedom of choice, and discourages critical thinking, caution, and reason. The second type of discipline is internal discipline, or self discipline. This is the ability to do what you want to do despite your subconscious or physical barriers not allowing you to do them. Self discipline is extremely useful, and as opposed to external discipline promotes freedom of choice and critical thought.
Self discipline is what is needed to do what you really want to do as opposed to what you think you want to do. Your subconscious will tell you to do things that it is programmed to do such as eating, bodily functions and reproduction. While some of these urges justified to keep you healthy and safe, others such as lust, greed, and laziness are not beneficial and are often contrary to what you actually want to do. This does not necessarily mean that the actions that these impulses want you to do are bad, it simply means that impulsive behavior is not necessarily good. It can be helpful to force someone to do something beneficial, but since these actions are pre-determined and somewhat arbitrary, they do not use reason. To determine which of these actions you should do, they should be analysed to determine if they are reasonable.
Self discipline then allows you to do the things you have determined you want to do, and to resist the urges that you disagree with. This is particularly difficult when impulses make you think you want the action. The easiest way to get someone to do something is to get them to like it. If someone likes something, they are less likely to question if they are being compelled to do it or if they actually want to. The fact remains though that this desire is induced and can be differentiated from naturally formed likes.
This is why the phrase “I want” is simplistic. It does not take into account that there are different types of “wants” and in a way different “I”s. Your subconscious may want to do something and will compel you to do it by making you desire it. That desire is chemically induced, and does not come from the rational part of your brain. In a way it is you who wants it but it is only a part of you. The other part of you, the one that isn’t programmed and can make it’s own decisions, may want something else. Self discipline is the ability for the conscious part of the brain to not be controlled by the unconscious part. The ability to withstand pain, desire, and even physiological constraints allows you to decide whether or not something that had been programmed to help you will actually help you. The brain might shut itself down to protect itself, but this may often lead to harm that you can’t avoid since you are not conscious. The subconscious does not question itself, it does not think ahead, and it does not necessarily know what is best for you. This is why you need to question it and decide for it, otherwise you are essentially letting someone else not only decide what is best for you, but also make you to want to do it.