The philosophical concept of identity is distinct from the more commonly understood notion of identity in psychology and the social sciences. In philosophy, identity concerns a specific relation: that x and y stand in this relation if, and only if, they are one and the same thing, or identical to each other (i.e., if and only if x = y). This is often referred to as numerical identity. In metaphysics, identity, derived from the Latin *identitas* meaning
"sameness," is the relation each thing bears only to itself. This notion gives rise to numerous philosophical problems, including the identity of indiscernibles—the question of whether two things sharing all their properties are, in fact, one and the same—and questions about change and personal identity over time.
Numerical vs. Qualitative Identity and Metaphysical Questions
It is crucial to differentiate between qualitative identity and numerical identity. Qualitative identity refers to similarity, such as two children having identical bicycles. Numerical identity, on the other hand, refers to being one and the same thing, like two children having the same mother. Philosophical discussions about identity primarily focus on numerical identity, which is the stricter concept. Metaphysicians and philosophers of language and mind explore profound questions related to identity, such as what it means for an object to be the same as itself, and whether identical things must always be identical, or necessarily identical.
Further questions arise regarding how an object can remain the same if it changes over time. For instance, is an apple at one time the same as that apple at a later time? The classic Ship of Theseus example illustrates this problem: if an object's parts are entirely replaced over time, in what way can it still be considered the same object? The law of identity was historically considered a "law of thought." Gottfried Leibniz provided a modern formulation, asserting that x is the same as y if and only if every predicate true of x is true of y as well. Leibniz's ideas have influenced the philosophy of mathematics, particularly the development of predicate calculus as Leibniz's law.
Identity and the Self: Locke's Consciousness-Based View
John Locke significantly contributed to the understanding of personal identity, or the self, by grounding it in consciousness, specifically memory, rather than in the substance of the soul or the body. In his 1689 work, *An Essay Concerning Human Understanding*, Locke's chapter "On Identity and Diversity" is considered one of the first modern conceptualizations of consciousness as the repeated self-identification of oneself. Through this identification, moral responsibility could be attributed to an individual, and punishment and guilt could be justified.
According to Locke, personal identity "depends on consciousness, not on substance" or the soul. We remain the same person to the extent that we are conscious of past and future thoughts and actions in the same way we are conscious of present ones. If consciousness is the "thought" that "goes along with the substance…which makes the same person," then personal identity is founded solely on the repeated act of consciousness. Locke argued that self-identity is not based on the soul, as one soul might contain various personalities. Nor is it founded on the body substance, as the body can change while the person remains the same. He noted that even animal identity is preserved in the identity of life, not substance, as an animal's body grows and changes. For humans, identity is based on consciousness.
This perspective leads to a problematic thought: since personal identity is based on consciousness, and only an individual can be aware of their own consciousness, external human judges may never truly know if they are judging and punishing the same person, or merely the same body. Locke contended that one can only be judged for the acts of the body, as this is what is apparent to all but God. We are only responsible for acts of which we are conscious. This forms the basis of the insanity defense, where one cannot be held accountable for unconscious acts. Locke's conception of personal identity, therefore, places it not in the body or substance, but in a "same continued consciousness," distinct even from the soul, as the soul might not be conscious of itself. For Locke, the body may change, but consciousness provides continuity, making personal identity rooted in consciousness.
