As we’ll see, a living being does not move itself from potency to act. This is true because a) a living being is moved into existence by something other than itself; b) a living being’s powers of movement are not brought into existence by the living being, itself; c) a living being’s powers of movement require the influence of the outside world to grow and strengthen; and d) a living being’s parts that move (e.g. a leg or arm) look to other parts of the same being for the source of their movement. Since points (a) and (b) are self-evident, we'll focus on points (c) and (d). Let’s investigate!
A living being is a composite of many different physical components. When a living being moves itself, one part of the being moves another part of the same composite being. In other words, one part of the body looks to another part for the cause of why it's moving or changing (e.g. the movement of an arm requires bio/electrical instructions from the brain). Further, the sources of movement within the body comes from something called a power.
PASSIVE POTENCY VERSUS ACTIVE POTENCY
Before we go on, though, it's necessary to first distinguish between passive potency and active potency. Not understanding the difference can be a source of confusion. When referring to potency, we’re typically referring to passive potency. As we said on the previous page, a passive potency can only be acted on, and it can only be moved into act by something else that is already in act.
An active potency (a.k.a. power), on the other hand, is a type of potency that already exists and, as such, is in act. However, a power is also in potency in that it's not always using its ability at any given moment. This is why it's called an active potency.
EXAMPLE OF AN ACTIVE POTENCY
Let’s give an example. At this moment, you have an arm with muscles that can lift 10 pounds. These muscles are real but are not being used at the moment -- but they could be. This is an example of a power that's in act but in potency to being used. When you decide to use your arm to lift 10 pounds, this is an example of a power that's in act and which has been deployed in act to lift 10 pounds.
In addition to being an active potency that can operate, function, and move things, your muscles also have a passive potency in that they can grow stronger or weaker. But this power can only be changed by a reaction, stimulus, or resistance from something external (i.e. resistance for muscles, stimulus for sense, evidence for the intellect, attraction for the will, etc.) So, by using external resistance (e.g. lifting weights), you can take the passive potency contained in your muscles and strengthen your muscles to lift 15 pounds instead of only 10 pounds.
Therefore, to the degree that a power has an ability to act on something, it's an active potency that's in act and which can act on other things. To the degree that the power is able to strengthen or diminish, it has passive potency. Even though the interplay between passive potency and active potency can be complex, both types of potencies ultimately look to something else that already exists to actualize them.
CREATION AND DEVELOPMENT OF AN ACTIVE POTENCY
If you wish to expand your understanding, we can clarify things further. There is a point where every power of a living being didn’t exist and was, in fact, a passive potency. To bring a power into existence, something already in act was required to move it from passive potency into act. An example would be parents that bring a child into existence through sexual intercourse.
At conception, genes are formed within the child which trigger the initial development of the brain, muscles, etc. These genes contain DNA which contain instructions on how cells are to divide and grow. Once these powers are brought into existence by genes, the powers of the young child in the womb are negligible. However, as these powers develop through further genetic instruction and the impact of the outside world which slowly acts on them by offering stimulus and resistance, the powers are able to develop and strengthen. When a power (e.g. a muscle) develops and becomes established by external action and stimuli, it becomes a source of its own activity.
While it’s true that a fully developed power becomes a source of its own activity, this is true only after it has been created and developed by activity that is outside the power. Therefore, all self-movement of a living being originates from something else that was already in act before it.
Do Living Beings Move Themselves?
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