An excerpt from a recent paper:
Incidental being is not an object of science because it is neither always and necessarily in the same condition, nor even for the most part so. The essential stability of knowledge depends upon an essential stability of being. The analogy used to show this is a remarkable example of the way in which a preliminary step prepares us for subsequent stages of the argument—truth which is ultimately prior is implicit in that which is prior to us.
Wisdom’s relation to incidental being is like that of a fancy-chef or house builder. A chef who cooks for taste will only incidentally make a healthy meal. The healthy nature of the food is already present in the ingredients. The nature of such food, as healthy, is incidental to the art of this chef. Here we have an anticipation of the way in which the underlying thing will possess attributes that are incidental to the essential nature of a being—the reason why a composite being is more than its essence. The example of the builder clarifies how this works in nature. A house builder does not make his materials absolutely. He may cut wood, but he does not make the wood itself. Further, lumber has certain properties as material, because it had a prior nature as a tree. Lumber retains a certain character and organization, although it has been reduced to a state of privation and elemental substantiality.
What does this have to do with knowledge and the nature of being? Nature does what it does always or for the most part. A tree will produce another of its kind. It will not always produce one of the same dimensions or of the same appearance. So there can be knowledge an oak, but not of the accidents of an oak (e.g., why it is this height). Such a science would be derivative and descriptive, rather than primary and explanatory. Yet an oak will always have accidents. The significance of this will be unpacked as the argument progresses.
 E.2; cf. Plato’s Gorgias. The chef cooks by a knack or experience, not art or knowledge according to Plato. This is contra a doctor who knows both the purpose (telos) of the body and the cause of health. Aristotle playfully turns the example on its head, asking us to consider the very nature of science. It is likely he does not consider the art of cookery an exemplar, but a derivative art (cf. Γ.4-5).