It is not enough to point out that Aristotle's thinking is not always systematic. For this observation to amount to anything would require an enumeration and clarification of the differences from systematicity in his writings, as well as a demonstration of the extent to which these characteristics of his writings are not merely the mode of presentation of a set of doctrines which is in itself systematic. (One way of reading the Nicomachean Ethics, once it has become obvious that the text cannot be a coherent set of asserted propositions, is to try to identify which of those propositions represent Aristotle's actual doctrine or "considered opinion," and which were only a imprecise scaffolding or dialectical counterpoint. This seems to me still to attribute too much systematicity to Aristotle.)
As Pseudonoma already pointed out, the first hold Aristotle gets on the good of human life in the Nicomachean Ethics is dialectical: there is no scientific grounding for the position that politics is a master art, or that it would belong to such an art to supply governing knowledge of the highest good, or that the ground of a city is higher than that of the individual. Furthermore, as I pointed out in answer, the entire passage bases its validity on the hypothesis that there is a highest human good at all, and it is only in the context of this hypothesis that it even makes sense to say what that highest good would be (whether scientifically or dialectically). This hypothesis is not a presupposition, but a consciously hazarded entrance into a question which it might do no good to pursue. The study of ethics cannot at its inception justify itself as something ethical--which puts the following limitation on the possible findings of ethics: if in the end ethics does some good, and if this is because it directs us to the highest good, then it follows that the good of human life must be accessible without the introduction of the certainty born of deliberation (which means, a fortiori, without ethics); or if ethics does no good, well, that has almost the same result.
But before I can explain my sense of the first book of the Ethics, there is still one more important imprecision to be enumerated...tomorrow.