I may post from time to time, God willing, on topics that will help Roman understand concepts that will better enable him to read his new book "A Shorter Summa". As I have no need to make this private I will deposit my comments here, on the blog.
One topic I briefly raised was the matter of "form." You will come across this term regularly in the Summa Theologicae (yes, my name should have an "e" on the end and I hope to fix it if I can).
Firstly, what is mean by "form"?
Form is what makes something what it is. It is what makes a tree a tree and an apple an apple.
The worst mistake we can make in thinking of "form" is that is is the shape of the object.
Take apples for instance. There are many varieties of apples in the world and even those of the same variety differ amongst themselves in their precise features. Yet they are all apples.
The form of an apple is "appleness." Appleness is what makes an apple what it is. If you were to try and draw "appleness" (what is common to all apples) you would find it impossible to do so.
This helps us to see that form is not the shape or outline of the object.
In an actually existing material object form is united to matter. Matter is the individuating principle. It is what makes it this apple this apple and not the one over there or the one next to it.
"Appleness" is a substantial form. The individual features a particular apple has when its substantial form is united to prime matter are its accidents or accidental forms.