In my book I mention 3 things that can be divided into essences and accidents.
First are universals. This is a coffee cup, and that is a wine glass. Both, however, are liquid containers. So, “liquid container” as a genus can be called an essence, and the type of liquid it is meant to contain or difference can be called each item’s individualizing accident. We can vary the accident without altering the essence.
Second are material particulars. I quote Thomas Reid:
All bodies, as they consist of innumerable parts that may be disjoined from them by a great variety of causes, are subject to continual changes of their substance, increasing, diminishing, changing insensibly.
When such alterations are gradual, because language could not afford a different name for every different state of such a changeable being, it retains the same name, and is considered the same thing.
Merely material particulars do not have accidents in themselves but may have them considered as for “convenience of speech.” For example, over time, the flowers painted on my coffee cup may have faded; but I still consider this vessel to be essentially “my coffee cup.” I don’t have to do it; but it’s convenient for all concerned. I could say “Could you wash my coffee cup?” to my cleaning lady and be understood both today and 2 years ago or hence.
(I might be able also to point to the painted flowers and say, “See, these are no accident! When I was shopping for a coffee cup, I specifically looked for one with purple flowers on it. It expresses my love for philosophy” (or something). In this case, as soon as the color has faded sufficiently, I will want to throw the cup away which would make it no longer “mine” and cause it to corrupt — lose its essence — thereby.)
Third are spiritual particulars, especially human beings. The difference is that a person’s accidents are not constructed by other people but are mind-independent.
Note two things in this connection: first, a person’s character and virtues and personal idiosyncrasies individualize him; yet all people have a character. One person is courageous, another is cowardly, so (1) both are humans and yet (2) both must definitely possess this cardinal virtue in some variable degree.
Second, as each coffee cup must have sides of some thickness, so each man must exhibit some courage. Yet if a cup becomes less thick from use, I am perfectly free to call it a different cup; yet a man who increases in courage maintains his personal identity whether I like it or not. What exactly personal identity consists in is a non-trivial question, but it clearly constitutes the difference between material and spiritual particulars.