Defending Life, Chapter 6 Summary:
(For previous chapter review, go here.)
In their quest to dehumanize the unborn, abortion advocates often ignore the all-important distinction between substance things and property things. Living things are substances that maintain their identities through time while property things, such as cars and machinery, do not. A property thing, like my car, is nothing more than the sum total of its parts. Change the motor or replace a tire, you technically have a different vehicle. There is no internal nature (or essence) that orders its development and grounds its identity through change.
By contrast, a substance maintains its identity over time and change. What moves a puppy to maturity or fetus to an adult is not an external collection of parts, but an internal, defining nature or essence. As a substance develops, it does not become more of its kind, but matures according to its kind. It remains what it is from the moment it begins to exist. A puppy does not become more of a dog as it matures. Consequently, a substance functions in light of what it is and maintains its identity even if its ultimate capacities (for example, the ability to bark) are never realized.
Put differently, a substance is an entity in which the whole is greater than the sum of its parts, and the whole contains the internal nature that gives it unity and cohesiveness. Substances maintain their identity through change, while property things do not. A substance will develop accidental properties (such as self-awareness, size, and physical structure) as it matures, but these properties are non-essential and can be changed without altering the nature of the thing itself. This is why a person can lose a body part and yet retain his personal identity through that change.
Applied to the pro-life case, the substnace view says that you are identical to your former fetal self. You were the same being then as you are now, though your functional abilities have changed. From the moment you began to exist, there's been no substantial change in your essential nature.