The metaphysical foundations of identity over time
The problem of identity over time has received a tremendous amount of attention since David Lewis' famous discussion of the ‘problem of temporary intrinsics’ in 1986. Ultimately, the problem amounts to giving a cogent explanation for how an object can change qualitatively while remaining numerically identical over time.
Theories of identity over time traditionally divide into two groups: (1) Four-dimensionalism, or perdurantism; (2) Three-dimensionalism, or endurantism. The four-dimensionalist holds that objects are identical over time in virtue of having temporal parts which exist at different times. The three-dimensionalist rejects the ontology of temporal parts, and instead holds that objects are identical over time in virtue of being ‘wholly present’ at more than one time.
I consider and reject the two most widely accepted four-dimensionalist theories, worm-theory and stage-theory. I argue that any four-dimensionalist theory will have an unfortunate commitment to the counterpart theory of modality, and consequently, that four-dimensionalism should be rejected. I further consider three three-dimensionalist theories, and argue that they all have unacceptable consequences.
I then argue that all of these contemporary theories fail for the same reason, namely, they fail to consider the metaphysical foundations of identity over time. These foundations consist of the concepts of unity and form, which are defined in terms of ontological dependency. Ultimately, it is only after taking into account these metaphysical foundations that a satisfactory theory of persistence will emerge.
The positive theory of identity over time that I propose is neo-Aristotelian in nature, and consequently, much of this project involves giving contemporary arguments for, and accounts of, a broadly Aristotelian ontology, with the greatest emphasis on substance, unity, and form. Ultimately, my project has a simple structure: first, clarify and motivate an Aristotelian ontology, and then employ that ontology to generate a more plausible theory of identity over time. The theory that I propose is consistent with three-dimensionalism in so far as I reject the ontology of temporal parts, but it shares little else with traditional three-dimensionalist theories.