An accidental feature is a feature that does not determine a material's identity. Accidental features contribute to a material's overall characteristics without being what makes the material the kind of material it is.
Take a stone. The stone has a certain mass, a certain shape, a certain temperature, a certain position. Most of these are accidental. The mass and shape and temperature can change without changing the stone into a different material. Drop the stone in cold water and the temperature drops; it is still the same stone. Break the stone in half and the shape changes; each piece is still the same kind of material. The features that change without changing the kind are accidental.
Accidental features still matter. They are the variables we measure when we want to track a particular material at a particular time, the data we collect when we want to know how a sample is doing. They simply do not, by themselves, settle the question of what kind of material we are looking at. That question is settled by the essential features. The accidental ones describe how the material is right now.
Question
If you break a stone in half, do you have a different material?
Reveal Merrill's answer →Hide answer
No. The shape of the stone changed, but its kind has not. Shape is an accidental feature; the stone is still the same kind of material in two pieces. The essential features, those that define what kind of material it is, have not changed.