Full Idea
If it be demanded why rhubarb purges choler, snow dazzles the eyes rather than grass etc., that these effects are performed by substantial forms of the respective bodies is at best but to tell me what is the agent, not how the effect is wrought.
Gist of Idea
To cite a substantial form tells us what produced the effect, but not how it did it
Source
Robert Boyle (The Origin of Forms and Qualities [1666], p.47?), quoted by Peter Alexander - Ideas, Qualities and Corpuscles 01.2
Book Reference
Alexander,Peter: 'Ideas, Qualities and Corpuscles' [CUP 1985], p.43
A Reaction
This is the problem of the 'virtus dormitiva' of opium (which at least tells you it was the opium what done it). I take Aristotle to have aspired to a lot more than this. He wanted a full definition, which would contain lots of information about the form.