He treats the subject of first philosophy in 1.1-4, its first principles of conceptualization and assent in 1.5-8, and his extended treatment of the objects of inquiry in books 2-10 concern various ontological, aitiological, and theological problems.
This last point becomes clearer as the central argument of the Ilahiyyat develops from the subject of metaphysics as being qua being; (45) through the presentation of primary notions concerning being, thing, one, necessary, truth, and the primary assents concerning the comprehensive division of existence into necessary existence through another and necessary existence in itself; (46) and the application of these primary principles to the categories, (47) to the transcategorical disjunctives--like universal and particular or cause and effect; (48) and to the aitiological and theological demonstrations of God's existence as the divine necessary existence in itself who is the ultimate uncaused efficient cause of existence, creator of all possible existents, and ultimate end for all creation.
In fact, given his criterion of unity of action, Aristotle could very well acknowledge the aitiological character of Thucydides's work, and still qualify it as a particular logos that lacks unity and is particular.
For even if they understand historia as aitiological zethesis, their function is to provide accurate records of ta genomena, (73) and experience seldom offers perfectly formed chains of events unencumbered by accidental and chance occurrences.