When thinking and talking about principle of subsidiarity I’ve tended to resort to using metaphors of size and space (i.e., nothing should be done by a higher or larger organization which can be done as well by a smaller or lower organization). But philosopher Brandon Watson explains why that is not really what subsidiarity is all about:
The subsidiarity principle is often paired with the principle of solidarity, and there is a real connection between the two. Solidarity is the active sense of responsibility of each person for each person; it therefore requires the active and free assumption of responsibility for others. Subsidiarity, on the other hand, is the assistance of actual people through intermediary organizations; it therefore requires the active recognition of the free responsibilities of others. When applied to life in general, they are closely associated with the virtues that uphold civic friendship and civic order, respectively. When applied to Christian life specifically, solidarity is a principle the purest expression of which is the Passion of Christ, while subsidiarity is a principle the purest expression of which is divine Providence, and we are called to exhibit both, in higher and purer forms than mere natural friendship and mere natural prudence require, because as Christians we are called to participate in both Christ’s Passion and God’s Providence.
In this light one can see that [Catholic moral theologian Meghan Clark] is quite right to reject the interpretation in which subsidiarity is just a way of saying that smaller is better; if one took the phrase “smaller is better” rather loosely, it could very well be applied to subsidiarity, but it’s also potentially very misleading. What Clarke misses, though, is that her own preferred way of speaking, “decisions should be made at the lowest level possible and the highest level necessary,” runs into the same kinds of problems. Subsidiarity is no more (and no less) about height than it is about size. If we don’t make too much of it (and understand “lowest level possible” and “highest level necessary” so that they end up being the same level, rather than two mutually exclusive levels), it can be an entirely reasonable approximation. But it’s not what subsidiarity is about.
What subsidiarity is about is recognizing what organizations express and further human personality and a truly human life in the most natural and basic and person-focused ways, and both not interfering with them to the extent that they do this and also actively furthering it. Subsidiarity will tend under common conditions to favor smaller organizations and certain levels of governance, but just as subsidiarity may actually require larger organizations to step in, or even to be created so that they can step in, so also subsidiarity may actually require that decisions be made at levels higher than necessary. Likewise, subsidiarity may at times require higher levels to make it possible for a lower level to make decisions that it would not otherwise be able to make. What really matters in subsidiarity is not size or level but active help for the true flourishing of each person through those institutions and organizations that make this flourishing possible. [emphasis in original]