1. Correspondent to _cause_ and _effect_, are POWER and ACT; nay, those and these are the same things; though, for divers considerations, they have divers names. For whensoever any agent has all those accidents which are necessarily requisite for the production of some effect in the patient, then we say that agent has _power_ to produce that effect, if it be applied to a patient. But, as I have shewn in the precedent chapter, those accidents constitute the efficient cause; and therefore the same accidents, which constitute the efficient cause, constitute also the _power_ of the agent. Wherefore the _power of the agent_ and the _efficient cause_ are the same thing. But they are considered with this difference, that _cause_ is so called in respect of the effect already produced, and power in respect of the same effect to be produced hereafter; so that _cause_ respects the past, _power_ the future time. Also, the _power of the agent_ is that which is commonly called _active power_. In like manner, whensoever any patient has all those accidents which it is requisite it should have, for the production of some effect in it, we say it is in the _power_ of that patient to produce that effect, if it be applied to a fitting agent. But those accidents, as is defined in the precedent chapter, constitute the material cause; and therefore the _power of the patient_, commonly called _passive power_, and _material cause_, are the same thing; but with this different consideration, that in cause the past time, and in power the future, is respected. Wherefore the power of the agent and patient together, which may be called entire or _plenary power_, is the same thing with _entire cause_; for they both consist in the sum or aggregate of all the accidents, as well in the agent as in the patient, which are requisite for the production of the effect. Lastly, as the accident produced is, in respect of the cause, called an effect, so in respect of the power, it is called an _act_. [Sidenote: An act is produced at the same instant in which the power is plenary.] 2. As therefore the effect is produced in the same instant in which the cause is entire, so also every act that may be produced, is produced in the same instant in which the power is plenary. And as there can be no effect but from a sufficient and necessary cause, so also no act can be produced but by sufficient power, or that power by which it could not but be produced. [Sidenote: Active and passive power are parts only of plenary power.]