Let the object which we suppose to begin its existence of itself be imagined, abstracted from the nature of all objects we are acquainted with, saving in its capacity for existence; let us suppose it to be no effect; there shall be no prevening circumstances whatever that affect it, nor any existence in the universe: let it be so; let there be nought but a blank; and a mass of whatsoever can be supposed not to require a cause START FORTH into existence, an dmake the first breach on the wide nonentity around;--now, what is this starting forth, beginning, coming into existence, but an action, which is a quality of an object not yet in being, and so not possible to have its qualities determined, nevertheless exhibiting its qualities?[Lady Mary Shepherd, An Essay upon the Relation of Cause and Effect, p. 35.]
That's a pretty hefty sentence, so let's break down the idea a bit. Hume (who is Shepherd's main target here) says that it is possible to imagine something beginning to exist without a cause. Shepherd argues against this view in this way (more or less):
(1) Suppose there to be an object that begins to exist without a cause.
(2) Beginning to exist is an action.
(3) An action is a quality (feature) of something that exists.
(4) The object that begins to exist cannot exist until it has already begun to exist.
(5) Therefore the action involved in beginning to exist is not the action of the object that begins to exist.
(6) This is to have a cause, which contradicts the supposition.
As she puts it a bit later:
But if my adversary allows thtat, no existence being supposed previously in the universe, existence, in order to be, must begin to be, and that the notion of beginning an action (the being that begins it not supposed yet in existence), involves a contradiction in terms; then this beginning to exist cannot appear but as a capacity some nature hath to alter the presupposed nonentity, and to act for itself, whilst itself is not in being.--The original assumption may deny, as much as it pleases, all cause of the existence; but, whilst in its very idea, the commencement of existence is an effect predicated of some supposed cause, (because the quality of an object which must be in existence to possess it,) we must conclude that there is no object which begins to exist, but must owe its existence to some cause.[Lady Mary Shepherd, An Essay upon the Relation of Cause and Effect, pp. 35-36.]