Are essences eternal and immutable?
In their physical existence, essences are not eternal, but created in time; but they may be called immutable, inasmuch as they remain while the substance lasts, for the accidents alone are changed. In their logical entity or intelligibility, essences are eternal and immutable, inasmuch as it ever is and ever was true, e.g., that an intellect supposes a simple substance, that a part is less than the whole, etc. 32. It is clear that we know the essences of the things which we make or invent ourselves, as of a watch, a table, etc. We also know the essences of many things in nature, as of a fruit, a tree, an animal, the intellect, etc.; else, we could have no science about such things, since science treats not of singular things but of the essences of things. Still there are many natural agents of which we do not know the specific essences, e.g., heat, electricity, magnetism, etc.; we know what they do, but not what they are.
