Background to the discussion of identity
The first point to note about the discussion of identity is that there are several terms, and consequently, concepts, that are used. They include the following:
(1) Human life
(2) Human being
(4) Human organism
It is important to understand the differences between these concepts.
Some hold that there may be said to be "human life" even if there is not a human being. On this view, a cluster of living cells -- say, some skin tissue -- is "human life", even if this cluster is not a human being. Others who make this distinction claim that a fertilized ovum or zygote, and also perhaps a pre-embryo, and also perhaps an embryo, and maybe an early fetus, are also "human life", but not human beings.
"Human being" is normally used to refer to a member of the species homo sapiens. It is, then, normally used as a biological classification. However, some hold that precisely because of this it is not the same as "person".
Those who hold this may argue that there are persons who are not human beings. (Note that persons are always "who" or "he/she", and not "that" or "it".) A god, for example, may be a person, and not a human being. Or an angel may be a person, and not be a human being. Or a Martian may be a person, and not be a human being. Or even an animal may be a person, and not be a human being.
Furthermore, some who argue for the distinction between "human being" and "person" argue that there are human beings who/that are not persons. (Note that human beings are normally "who" or "he/she", and not "that" or "it"; but there may be contexts in which they are "that" or "it", according to some). The following are sometimes considered to be human beings who are not persons:
(a) Zygote (fertilized ovum after ) (may divide)
(b) Pre-embryo (up to six days, when implantation normally occurs) (set of undifferentiated cells, any one of which could give rise to an embryo under certain circumstances -- they are totipotent) (may divide) (may combine with another embryo)
(c) Embryo first stage (from six days to fourteen days) (cells are differentiated; from fourteen days to eight weeks) (can divide)
(d) Embryo second stage (from fourteen days to eight weeks) (cannot divide)
(e) Fetus (after eight weeks until birth)
(f) Anencephalic infant (due to severe congenital malformation of the brain resulting in the possession of at most rudimentary cerebral hemispheres, the infant is in a permanent vegetative state)
(h) Comatose human being
(i) Child or adult human being whose upper brain (consisting of the cerebral hemispheres, which contain the neurophysiological basis of consciousness, as well as higher mental functions such as self-consciousness, deliberation, thought and memory), has been damaged or destroyed or removed, resulting in the loss of consciousness and all higher mental functions
(j) Same as (g), except that the damage (and it can only be damage here) has the result that while consciousness is preserved, all higher mental functions are lost
(k) Adult human being who suffers e.g. from severe Alzheimer's Disease, with the result that
(l) Human corpse
(This list is not exhaustive.)
Those who argue for a distinction between "human being" and "person" normally argue that there are certain properties that must be possessed by a human being in order to be a person. These include:
(1) Is conscious
(2) Has preferences
(3) Has conscious desires
(4) Has feelings
(5) Feels pleasure and pain
(6) Has thoughts
(7) Is self-conscious
(8) Has rational thoughts
(9) Has a sense of time
(10) Remembers its/his/hers own past actions and mental states
(11) Can envisage a future for it-/him-/herself
(12) Has non-momentary interests, involving a unification of desires over time
(13) Rationally deliberates
(14) Takes moral considerations into account when choosing between possible actions
(15) Has character traits that undergo change in a reasonably non-chaotic fashion
(16) Interacts socially with others
(17) Communicates with others
It is normally granted that a human being who has all of these properties is a person. However, there is much debate over whether a human being must have all of these properties in order to be a person, or whether only some are necessary. Some also hold that some of these properties are special, such that possessing it is sufficient for a human being to be a person. For example, some hold that any human being who/that is self-conscious is a person.
(It is important to note here that those who hold this often hold that anything that has this special property is a person. So if a primate or a Martian is self-conscious, then he/she/it is a person.)
Some go further than this and argue that merely having the capacity for all or even some of these properties is sufficient for a human being to be a person. It is important here to consider the difference between the following, however:
(1) Capacity for property x (e.g. consciousness) that is exercised
(2) Capacity for property x that is unexercised
(may perhaps be possessed by human being with a brain with certain complex neuronal connections completed at a time when the human being is not conscious)
(3) Potential to have capacity for property x
This distinction introduces the distinction between potentiality and actuality (in the form of potentially having a capacity vs. actually having a capacity.) This distinction is important. A citizen who is 18 is actually legally entitled to vote. A citizen who is 15 has, or may have, the potential to be 18. But a citizen who is 15 is not actually entitled to vote. Similarly, a caterpillar is, or may be, potentially a butterfly. But a caterpillar is not actually a butterfly.
In the case of the distinction between human being and person, the actuality/potentiality distinction may arise in the case of properties of persons and human beings, capacities for properties of persons and human beings, and also human beings and persons.
In the case of the actuality/potentiality distinction as applied to human beings and persons, potentiality is normally distinguished as follows:
(1) Passive potentiality
(2) Active potentiality
Here it is said that e.g. an unfertilized egg and a spermatozoon only have passive potentiality to become a human being, or a person. A fertilized ovum, or zygote, however, has active potentiality to become a person or a human being (that is, if he/she/it is not already considered a human being.)
So far only "human being" and "person" have been considered. However, some hold that "human organism" is distinct from both, and that there are human organisms that are not human beings or persons. Examples of human organisms that are not human beings might be zygotes or pre-embryos or embryos or even fetuses. Furthermore, some hold that adult human organisms may not be human beings or persons, in the sense that it is possible that there exists one human organisms and two human beings or two persons. The case of dicephalic twins, for example, is considered to reveal that there can be one human organism and two human beings or persons (if it is accepted that here there is only one human organism).
The concept of an "individual" is distinct again from the other concepts, although this is often invoked in order to remain neutral between the others. It can mean an individual human organism, human being or person, or an individual zygote or pre-embryo or first-stage embryo or second-stage embryo or fetus.