Boundaries help us to define what is not on our property and what we are not responsible for. We are not, for example, responsible for other people.  In short, boundaries help us keep the good in and the bad out.  Sometimes, we have bad on the inside and good on the outside. In these instances, we need to be able to open up our boundaries to let the good in and the bad out.  

Boundaries are not walls. But in every community, all members have their own space and property.  The important thing is that property lines be permeable enough to allow pass and strong enough to keep out danger. 

Boundaries are anything that helps to differentiate you from someone else, or show where you begin and end.  The most basic boundary that defines you is your physical skin. The most basic boundary-setting word is no. It lets others know that you exist apart from them and that you are in control of you. Setting boundaries inevitably involves taking responsibility for your choices.  

Setting limits on others is a misnomer.  We can’t do that. What we can do is set limits on our own exposure to people who are behaving poorly; we can’t change them or make them behave right.  The other aspect of limits that is helpful when talking about boundaries is setting our own internal limits.  We need to have spaces inside ourselves where we can have a feeling, an impulse, or a desire, without acting it out.  We need self-control without repression.  We need to be able to say no to ourselves.

Henry Cloud, John Townsend writing in Boundaries