I've heard some people say that there are two types of instrument: secondary agents and tools.
A secondary agent is something that accomplishes a task when the agent wields it. So we CAN say sentences such as "the knife cut the paper," "the boat crossed the river," and "the oven cooked the muffins."
A tool is something that helps an agent accomplish a task himself/herself. We CAN say that we eat with a fork, but we CAN'T sensibly say "The fork ate the broccoli." We CAN say that we planted a tree with a shovel, but we CAN'T sensibly say that "the shovel planted the trees".
How hard and fast is this distinction? Have counterexamples ruled it out in the literature, is it still an open question, or is it considered established semantics?