This follows on from yesterday's post on form.
It may have been noticed that the examples I provided came from the natural world - apples and trees.
Sometimes, when starting out, it isn't unusual for the concept of form to be explained by examples which consist in things made by man. Such as shoes or hats.
The form of shoes, as you would have no doubt guessed, is shoeness. Shoeness is what makes a shoe a shoe.
Now there is an important difference between the forms of appleness and shoeness. Shoeness is what we call an artificial form. Appleness, on the other hand is a substantial form as mentioned previously.
The other concept touched on briefly below was that of prime matter. Prime matter is what a substantial form unites itself to. When it does so it turns into a particular apple or tree.
Prime matter, as such, does not exist. Prime matter is matter without any form whatsoever. It is pure potency. That means it has potency for any form (potency is "lack but can have"). It can potentially be everything. But being potentially everything it is in fact, nothing. Matter cannot exist without form. It needs to be actualised by some form. Of course, matter actualised by a form still retains a limited range of things that it can turn itself into. It still has some potency. For instance a piece of wood is potentially a pile of ashes. But it isn't potentially a muddle of water. Unlike prime matter it has a limited range of potencies.
Restricting ourselves to a consideration of those things which are material, their substance is their substantial form united to matter.
Please note what is meant by "matter" here. In the Glossary to "Summa of the Summa" philosopher Peter Kreeft says:
"matter: the principle in a thing's being by which it is able to be determined by form; potency as vs. actuality. In modern parlance, the word refers to actual, visible, formed things (chemicals, molecules) but in Thomistic and Aristotelian parlance "matter" is not itself observable or even of itself actual. It not a thing but a metaphysical principle or aspect of things, which together with form explains change, as the actualizing (in-form-ing) of potency (matter)."
This is important to ensure the word "matter" is not bringing up the wrong connotations.
So for material things:
substance = substantial form + matter
Some substances such as angels are pure forms not united to matter but we may be getting ahead of ourselves.
A substance is something that exists of itself rather than in something else.
What do we mean by this?
Earlier I mentioned accidents. Accidents are something's individual properties. It is a result of form being united to matter. All apples have the form of appleness but this particular apple may be such a shade of color or be a particular size or shape. No two apples are the same in other words. The matter individuates the form.
Accidents inhere (exist in) a substance. They are upheld or supported by the substance they exist in. An accidental change does not affect the substance. Putting a dent in the apple doesn't mean it ceases to remain an apple. However a change in substance will result in an change of accidents which inhere in the former. For instance if I burn a piece of toast I am making a substantial change (bread into carbon) with the necessary accompanying change in accidents.
A substance is of the opposite nature. It exists by its own being not in something else.
If you are thinking ahead you would have already noticed that the exception to this is the transubstantiation of the bread and wine into the body and blood of Christ in the Sacrifice of Mass. Two miracles are taking place here at the same time.