Accident, from the Greek συμβεβεκός (symbebekos), as used in philosophy, is an attribute which may or may not belong to a subject, without affecting its essence. The use of accident has been employed throughout the history of philosophy with several distinct meanings.
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In Aristotle's theory of the substance of objects, the concept of accident plays an important role in clarifying what he does not mean by substance. For Aristotle, accidents are the perceptible qualities of an object such as its color, texture, size, shape, etc.; as he states, things which do not affect the essence of the object are accidents. For example a chair being wood, metal, or plastic, is an accident, for it is still a chair regardless of the material it is made of.
St. Thomas Aquinas employed the Aristotelian concepts of substance and accident in articulating the theology of the Eucharist, particularly Transubstantiation. In summary, the accidents of the bread and wine do not change, but their essences change from bread and wine to the Body and Blood of Christ.
In modern philosophy, an accident (or accidental property) is the union of two concepts: property and contingency. In relation to the first, an accidental property (Greek sumbebekos) is at its most basic level a property. The color "yellow", "high value", "Atomic Number 79" are all properties, and are therefore candidates for being accidental. On the other hand, "gold", "platinum", and "electrum" are not properties, and are therefore not classified as accidents.
Aristotle made a distinction between the essential and accidental properties of a thing. An accidental property is one which has no necessary connection to the essence of the thing being described. A trivial example may help to illustrate the distinction. It is an essential property of bachelors that they are unmarried, but it is an accidental property of bachelors that they have brown hair. This is because it is logically impossible to find a married bachelor anywhere in this or any other possible world, and therefore the property of being unmarried is a necessary or essential part of being a bachelor. On the other hand, brown hair is a contingent or accidental property of bachelors since some bachelors have brown hair and others do not. Even if for some reason all the unmarried men with non-brown hair were killed, and every single existent bachelor had brown hair, the property of having brown hair would still be accidental, since it is the case that in some possible world, a bachelor could have hair of another color.
Aristotle addressed 10 different categories in his ontology, which could include categorization of different types of accidental properties. With sumbebekos being a quality not needed but accidental to a being, akin to unspecified attribute. In relation to the second, an accidental property is a specific subset of properties. Some members of the set of properties are argued to be essential (or necessary) to the object and are not categorized as accidental, such as "Atomic Number 79", while other properties are non-essential (or contingent) to the object and are categorized as accidental, such as "yellow" and "high value". This philosophical usage is defined more technically in modal logic, and due to an increasing focus on linguistic rigor in the last century, has been sharply separated from many other senses of the word "accident".
There are two opposed philosophical positions that also impact the meaning of this term: