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In this article, St. Thomas distinguishes between immutability that comes from extrinsic power and immutability that comes from intrinsic potentiality.
In the first way, all things are equally mutable, for, God could cease upholding whatever he wanted in being.
In the second way, there is a difference between the different "classes" of being.
First, we have the corporeal beings of the "inferior bodies." These have a composition of matter and form and thus have the highest degree of mutability. In this, there is BOTH a substantial mutability (e.g., wood can be burned into ash) and an accidental mutability (e.g., a black rock can be painted blue).
Yet, St. Thomas makes a further distinction within "accidents." For, there are "accidents" in the general sense and "properties" in a special sense. "Properties" are distinguished from general accidents in that they flow from the essence, whereas other accidents do not. Thus, in man, risibility (i.e., the ability to laugh) is a property that flows from our rational animality (i.e., our essence). These accidents flow from the subject and thus cannot (physically) be separated from the subject.