"This is a dog" is a statement identifying an item (substance) as a dog. This means the dog has all the characteristics necessary to be a dog, whether those be that it has four legs and barks or that it has the DNA sequence inside certain parameters, you've just stated that the substance has those characteristics--which are called Essential properties by philosophers. That doesn't, however, specify that the substance (the dog) is brown, as "brown" is (presumably) not a necessary characteristic for something to be a dog. The browniness of the dog is what we call an Accidental property, whilst the four-leggedness is an Essential property, if you use the first definition of a dog that I gave.

"This is My Hand" says that the substance is owned by me and is a hand. You may observe that my hand is orange right now, but to that I'll simply reply "Yes, but it's not My Hand because it's orange, it's orange due to a slight mishap with a highlighter. The orange, you see, is simply an Accident."

"Do you have accidents of this sort often?" you ask.

"Never mind that!" I say. "This is My Orange Hand!" I've just defined that substance as a hand owned by me and orange-coloured. Now if I wash my hand it loses one of it's Essential properties, not simply an Accidental properties. Orange is now a necessary condition for it's being what it is.

However: they're the same object, but for one the colour is an Accidental property and the other it's an Essential property. Therefore, Accidental properties and Essential properties are simply in the terms you use to identify an item and have no empirical distinction.

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