In the sciences, the word “mechanism” does not mean that the thing we are talking about is a series of pivots, glides, and rigid drivers that push or pull or twist each other about. A mechanism is anything that could be reconstructed or commanded by us. No one might have any idea how an atom moves from one orbit to another, but if we can make it do so on command or in circumstances under our control (i.e. experiments), then we can call it a speak of it as a mechanism.
Taken in this sense, the opposite of a mechanism is what determines its own action, i.e. first what is alive, then what is free, and God most of all. To the extent that the sciences are mechanistic, the scope of their universality is limited in a relative way by life and humanity, and in an absolute way God. And so one of the following three must be false:
1.) The sciences are experimental.
2.) The sciences treat of all that can be known in the natural world.
3.) Life, human self-determination, and God can be known to act in the natural world.