Since at least the time of Aristotle, philosophers have debated what it is that constitutes an individual person or thing. What makes it a unity, numerically one? What distinguishes it from everything else?
Individuation is related to the metaphysical problems of constitution, composition, colocation, essentialism, and identity.
Given two equal amounts of matter, they are distinguished by their shape or form. Given two things with identical form, they are individuated by being embodied in different material.
In information philosophy, identity depends on the total information in an object or concept.
We distinguish the intrinsic information inside the object (or concept) from any relational information with respect to other objects that we call extrinsic or external information. We can “pick out” the intrinsic information as that which is “self-identical” in an object. The Greeks called this the πρὸς ἑαυτο – self-relation. or ἰδίος ποιὸν, “peculiar qualifications” of the individual.
Self-identity, then, is the simple fact that the intrinsic information and the extrinsic relational or dispositional information are unique to this single object. No other object can have the same disposition relative to other objects. This is an absolute kind of identity. Some metaphysicians say that such identity is logically necessary. Some say self-identity is the only identity, but we can now support philosophers who argue for a relative identity.
To visualize our concept of information identity, imagine putting yourself in the position of an object. Look out at the world from its vantage point. No other object has that same view, that same relation with the objects around you, especially its relation with you. Now another object could have intrinsic information identicality. We will identify a very large number of objects and concepts in the world that are intrinsically identical, including natural and artifactual kinds, which we may call digital kinds, since they are identical, bit for bit. This is relative identity.
In 1947, Ruth C. Barcan (later Ruth Barcan Marcus) wrote an article on “The Identity of Individuals.” It was the first assertion of the so-called “necessity of identity.” Her work was written in the dense expressions of symbolic logic, with little explanation.
Five years later, Marcus’s thesis adviser, Frederic B. Fitch, published his book, Symbolic Logic, which contained the simplest proof ever of the necessity of identity, by the simple mathematical substitution of b for a in the necessity of self-identity statement (2).
(1) a = b,
(2) ☐[a = a], then
(3) ☐[a = b], by identity elimination.