to run only if he is in good condition, completely poised, in full readiness.
In a position to . . . , this means first: he is fit for it. Yet not simply this, but at the same time it also means: he ventures himself, has already become resolved. To actually be capable is the full preparedness of being in a position to, which lacks only the releasement into enactment, such that when this is at hand, when it has imposed itself, this means: when the one who is capable sets himself to work, then the enactment is truly practice and just this. It is nothing other than setting oneself to work—ἐνέργεια (ἔργον: the work or the product).
Now it becomes clearer how the actuality of δύνασθαι is to be comprehended through ἔχειν, having and holding, namely as holding oneself in readiness, holding the capability itself in readiness. This being held is its actual presence. In the example mentioned earlier, the potter who had lost both hands, the moment of passing beyond, of going over, is in a certain manner no longer at hand; the being held is no longer complete; the readiness is interrupted.
If you have followed this entire clarification of the essence of that which is capable and actually present with a continual view toward the phenomenon (the runner immediately before the start), then the "definition" which Aristotle now gives for δυνατὸν εἶναι may no longer be foreign to you.
1047a24-26: ἔστι δὲ δυνατὸν τοῦτο ᾧ ἐὰν ὑπάρξῃ ἡ ἐνέργεια οὗ λέγεται ἔχειν τὴν δύναμιν, οὐθὲν ἔσται ἀδύνατον.
"That which is in actuality capable, however, is that for which nothing more is unattainable once it sets itself to work as that for which it is claimed to be well equipped."
Here we have again one of the unprecedented and determining essential insights, through which Aristotle for the first time illuminates a previously obscure realm. In this concise statement, every word is significant. With Aristotle the greatest philosophical knowledge of antiquity is expressed, a knowledge which even today remains unappreciated and misunderstood in philosophy.