In their book Extreme Ownership, Jocko Willink and Leif Babin talk a lot about their claim that discipline equals freedom. While the book explains quite well what that means, I would like to offer an analogy which might be helpful in order to visualize the statement and therefore clarify it even more.
Imagine we are standing on the top of a mountain with a ball in our hands. Once we drop the ball, it immediately begins to roll down the slope until at has arrived at the valley (assuming it did not get caught by some obstacle along the way).
Now the ball will not take an arbitrary path down the slope; rather, it will take the path of least resistance. The path of least resistance has a remarkable property: It is usually unique (unless certain symmetries are given, which is not relevant to our point). This means that the path of the ball is completely constrained and predetermined: There are no options for other paths the ball could take.
Essentially the same is true for us humans: Without discipline, we will always take the path of least resistance. Again, it is usually unique: There is always one way in which we can minimize the resistance we are confronted with. For instance, when we are lying in bed, the one way to minimize resistance is to simply don’t move at all and just stay in bed.
Things change, however, once we can come up with a little bit of discipline; we are then no longer confined to the path of least resistance. Instead, we can deviate slightly from it, since discipline means nothing else than being able to push against or even overcome resistance. For instance, we can get out of bed and make ourselves a cup of coffee; or we can get out of bed and work out.
And the more discipline we can summon up, the more we can deviate from the path of least resistance. While our path was unique as long as we did not have any discipline whatsoever, we have now opened up other paths we can take, depending on our discipline: We are no longer confined to a single path, but have created options for taking other paths, and in general, the bigger our discipline, the more options we will have (since our level of discipline determines the level of resistance we can overcome).
But having options at our disposal is precisely the definition of “freedom”: We can now choose which path to take; alternative paths have become available to us. Therefore, discipline equals freedom.