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 composition, colocation, 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.

It was the general opinion of scholars for many centuries that Aristotle claimed matter (hyle) is what individuates a form or essence. Aristotle was openly skeptical about the independent existence of his mentor Plato's Ideas in his Theory of Forms (eidoi). But many commentators in the past several decades have shown that Aristotle ultimately came around to believe that an immaterial Parmenidean "being" or "essence" (einai) is also involved.

See J. Lukasiewicz, E. Anscombe and K. Popper (1953), Lloyd, A. C. (1970), Regis, E. (1976), Cohen, S. M. (1984), and Whiting, J. E. (1986).

Although some scholars argue for form instead of matter, information philosophy and modern biology show that both form ("information") and matter ("stuff") are always needed.

In his metaphysics Aristotle sought to understand "being qua being." Can there be a form without matter? Surely form without matter is empty and invisible. Matter without form is impossible, but if some material is merely formless or shapeless, it contains no valuable information.

Information philosophy notes that information is neither matter nor energy, though it needs matter to be embodied and energy to be communicated. Unlike matter-energy, information can be created and destroyed. The material universe creates it. The biological world creates it and utilizes it. Above all, human minds create, process, and preserve information, the sum of human knowledge that distinguishes humanity from all other biological species and that provides the extraordinary power humans have over our planet.

Information is the modern spirit, the ghost in the machine, the mind in the body. It is the soul, and when we die, it is our information that perishes. The matter remains.

Speculations about the mother (mater) providing formless matter for a child and the father (pater) providing the providing the form (pattern) in his seed (σπερμα) show that Aristotle knew both matter and form are needed to create an individual. At Metaphysics 1033b, he says, everything must "be partly one thing and partly another; I mean partly matter (hyle) and partly form (eidos)."

It is tempting to associate matter with Aristotle's material cause and form with his formal cause. We know he sometimes claimed one and sometimes the other as individuating, but everything consists of both.

At Metaphysics 1034a 8), he says Callias and Socrates are identical in form (man), but different because their matter is different. But at Metaphysics 1041b 8, he says, "Thus what we seek is the cause (i.e., the form) in virtue of which the matter is a definite thing; and this is the substance (ousia) of a thing.

Ancient religions described immaterial souls coming to earth to become embodied as material individuals. Did they bring a personal identity with them? Scholastics argued that all angels, who are not material, cannot be easily differentiated. They could all be colocated in the same place at the same time, on the head of a pin, for example.

Was Socrates' soul before his instantiation in material already Socrates? We have clear evidence that some Greeks thought not. Others wanted the immortal soul of Socrates to survive death. Consider this passage from Stobaeus:

So too in general when it comes to substance, to hold that we are the same as our substances seems unconvincing. For it often comes about that the substance exists before something's generation, before Socrates' generation, say, when Socrates does not yet exist, and that after Socrates' destruction the substance remains although he no longer exists.

(Stobaeus (I,177,21 - 179,17, in The Hellenistic Philosophers, Long and Sedley, v.1, p.168)

Aristotle, though he was critical of the Platonic forms (eidos or ideas), noted the importance of form as completing the individual. He notoriously used the term we usually translate as "substance" (ousia) in conflicting ways, sometimes talking of form as an essence (einai or being) as a "primary substance," (proten ousian) for example,

(by "form" I mean the essence of each thing, and its primary substance)

（εἶδος δὲ λέγω τὸ τί ἦν εἶναι ἑκάστου καὶ τὴν πρώτην οὐσίαν）

(Metaphysics, VII, vii, 1032b)

Stoics, like Chrysippus, argued that matter is the basic "underlying substrate" (υποκειμενον). That which identifies a "peculiarly qualified individual" (ιδιοσ ποιον) is a unique bundle of qualities or properties that come with the pneuma, a combination of air and fire that is approximately the earlier Greek (psyche or soul.

Academic Skeptics mocked the Stoics as seeing two things as "colocated," occupying the same place at the same time. The paradox of the lump of clay and the statue was a prominent example. This puzzle can be resolved by noticing that the two things are simply matter and form, which are almost always colocated.

Scholastic discussions ranged from Aquinas, who followed Aristotle making matter the principle of individuation, to the last great Scholastic, Francisco Suárez, whose principle of individuation included both matter and form, the total of information in an entity as we would say in information philosophy.

Like most problems in metaphysics, individuation has been analyzed and debated with close attention to words and concepts.

Information philosophy identifies abstract immaterial form as the information needed to specify exactly how to create an identical copy of a thing. In standard usage, the word form refers to an outer two-dimensional surface, that part of something that is most easily perceived. But information philosophy also needs the internal material parts - the elementary particles, the atoms, the molecules, etc. their instantaneous positions over time, their interactions with each other, and, in the case of living things, the communications of their component parts with one another and with other beings.

For abstract entities that contain no material substance, we can ask what could individuate them - two circles with the same radius, for example. If they are located at different places in space, that would work. But does this require their material embodiment, as ink on paper?

What about a circle that is in a single place, should we distinguish its temporal parts diachronically and ask whether the circle at t=0 is the same circle at t=1? This is a metaphysical problem known as persistence.

And we now know that every organism, even the simplest single-cell bacteria, archaea, and eukaryotes are unique individuals.

From the very earliest proto-life forms that could duplicate themselves, only some duplicates were exact replicas. As Jacques Monod pointed out, perfect reproductive invariance would proliferate a species, but without a modest number of random variations, there would be no evolution.

Perfect copies would be identical, differing only in their physical locations. A variation in their information content produces two intrinsically different individuals.

The most complex organisms, eukaryotic cells and multicellular organisms, use the deliberate randomization of chromosomes in sexual reproduction to produce essential variety in the gene pool. Even when a cell divides to produce two individuals that are genetically alike, the development process introduces variations that are not inheritable, but that ensure adults are unique individuals, because their information content differs.

The principle of individuation in biology is a combination of genetic and epigenetic differences in the information content of individuals. It is the form that differentiates them, not the specific material they are made of. We are different individuals because of chance events, from our first zygote stage to our last breath, that change our information content. Here the change is growth, with a high degree of preservation of the vital information. In higher organisms, what is preserved is learned information - recordings of experiences.

The material content of any organism also is in a state of continuous change, as food (matter with low entropy and high free energy) moves through an organism. It is the comparatively stable, but constantly growing, information content embodied in the material that we recognize as the organism.

Very few cells in a multicellular organism have lifetimes close to the life on an individual. In humans, some neurons and egg cells that do not reproduce can last a lifetime, sperm cells last only a few days, skin cells a few weeks, red blood cells a few months, and white blood cells a year or so. The stem cells that form new blood cells and epithelial cells in skin and the gastrointestinal tract can last a lifetime.

On average, all the material at the atomic and molecular level in a human body is replaced every seven or eight years, yet we persist as the same person over our lifetime. What philosophers of mind describe as the continuity of memory or consciousness, information philosophy sees as the stored information in the ERR (Experience Recorder and Reproducer).