I find this a confusing and difficult topic, but I’ve read some good explanations in these sources:
Brian Davies: The thought of Thomas Aquinas
Brian Davies: Aquinas
Eleanore Stump: Aquinas
Robert Pasnau: The Philosophy of Thomas Aquinas
Etienne Gilson: The Philosophy of Thomas Aquinas
My understanding from reading Aristotle and Aquinas is that ‘form’ is the organising principle that makes something organised in terms of its nature and structure. Aristotle and Aquinas though had a view of matter different than modern science; matter was not made of atoms or subatomic particles. However, the form of something is the configuration or structure that makes something what it is; for example, the structured configuration of the water molecule as H2O is what makes water ‘water.’ This form of explanation is still relevant if we talk about complex systems, particularly living ones, whose behaviour can’t be reduced simply to the rules or laws relating to its individual components.
