See Book Delta [W.D.Ross's translation of Aristotle’s Metaphysics (1924), from The Complete Works of Aristotle: The Revised Oxford Translation, edited by Jonathan Barnes (1984)]
[1013a24] We call a cause (1) that from which (as immanent material) a thing comes into being, e.g. the bronze of the statue [...]. (2) The form or pattern, i.e. the formula of the essence [...]. (3) That from which the change or the freedom from change first begins, [...] and in general the maker a cause of the thing made and the change-producing of the changing.
We have here in play the concepts of an agent and of power, i.e. a capability to produce an effect [see Actuality and Potentiality for the key Aristotelian distinction, between potentiality (dunamis) and actuality (energeia)].
[1013b4] These, then, are practically all the senses in which causes are spoken of, and as they are spoken of in several senses it follows that there are several causes of the same thing, [...] e.g. both the art of sculpture and the bronze are causes of the statue not in virtue of anything else but qua statue; not, however, in the same way, but the one as matter and the other as source of
"Movement" must be understood here in a general sense, as synonim of "change".
[1013b17] All the causes now mentioned fall under four senses which are the most obvious. [...] The semen, the physician, the man who has deliberated, and in general the agent, are all sources of change or of rest.
[1013b29] [...] But besides all these varieties of causes, [...] some are called causes as being able to act, others as acting, e.g. the cause of the house’s being built is the builder, or the builder when building.
Thus, the agent is an individual: "the sculptor causes the statue, in another sense Polyclitus causes it, because the sculptor happens to be Polyclitus" and, more specifically, a human being (we assume that no other animals are able to produce sculptures).
But not a "plain" man, but "Polyclitus the sculptor", i.e. a man endowed with a specific power : the power to produce a sculpture.
[1014a16] all [the causes] may be taken as acting or as having a capacity.