The paper proposes a novel understanding of how Aristotle’s theoretical works complement each other in such a way as to form a genuine system, and this with the immediate (and ostensibly central) aim of addressing a longstanding question regarding Aristotle’s ‘first philosophy’—namely, is Aristotle’s first philosophy a contribution to theology, or to the science of being in general? Aristotle himself seems to suggest that it is in some ways both, but how this can be is a very difficult question. My answer is in some respects a version of one that goes back at least to the middle ages—i.e., that first philosophy is concerned with the gods (and to that extent offers a theology) because the gods are causes and principles of beings precisely insofar as they are beings. The more original aspect of my position lies in my claim that the sort of tension found in the Metaphysics is likewise to be found in many of Aristotle’s physical works. Thus, for example, the De caelo is (I argue) concerned generally with natural beings (= beings susceptible of change), but its discussions are focused largely on the heavenly bodies and the Aristotelian elements insofar as they admit of change with respect to place. Here I claim that the particular objects of discussion are dealt with precisely because they are causes and principles of natural beings as such. Something similar goes, I claim, for the De generatione et corruptione, the general concern of which is a particular species of natural being—i.e., natural beings susceptible of generation and corruption. In this way, I argue, Aristotle successively deals in his theoretical works with those causes and principles of (say) a horse which attach to it insofar as it is a being, those causes and principles of a horse which attach to it insofar as it is a natural being, those causes and principles of a horse which attach to it insofar as it is a perishable natural being, and so on for the lower genera under which the species horse is subsumed.