Will you be the same person ten years from now? You might be dead, or have some kind of brain injury or dementia that has taken away much of your mental functioning. Is a demented or unconscious self still you?
These questions raise the traditional philosophical problem of personal identity, concerning what makes people who they are. Philosophers usually address this question using thought experiments about imaginary happenings such as brain transplants and teleportation. But such thought experiments are about as reliable a source of true conclusions as religious texts and Fox News. A more scientific approach to the self can better illuminate problems of personal identity.
My multilevel mechanisms theory of the self understands persons as a complex system based on interactions at four levels: molecular, neural, mental, and social. A mechanism is a combination of connected parts whose interactions produce regular changes. For example, a bicycle has parts such as the handle bars, frame, pedals, chain, and wheels whose connections and interactions with your body enable you to ride it down the street.
Mechanisms change when they get new parts such as a wheel to replace a broken one, or when their connections and interactions alter to produce different changes, for example when a bicycle chain gets loose making it hard to pedal. The identity of a mechanism is not all-or-none, but instead is a matter of degree depending on how much the parts, connections, and interactions have altered. Similarly, there is no simple answer to the question of whether you are the same person you used to be, because it depends on changes in four levels of mechanisms.
Your molecular mechanisms have probably only changed a bit in the past ten years. Barring mutations, you still have the same genetics based on DNA, but you have probably had some epigenetic changes in the chemical attachments that affect gene expression. You still have roughly the same neurotransmitters, but stress, depression, or good fortune may have affected the operation of ones such as serotonin and dopamine. Aging, maturing, or medication may also have affected levels of hormones such as estrogen and testosterone.