This paper aims at showing that Philo’s reflection on the possibility and content of the knowledge of God should be situated within its immediate philosophical environment, that is, mostly in the context of Stoic theology and the debate surrounding it. My main purpose is to show that acknowledging the Stoic orientation of Philo’s discussion of God casts light on Philo’s view of the knowledge of God. Although Philo repeatedly states that the essence and nature of God is not graspable (kataleptos) by a human mind, every man has to nevertheless acknowledge one of God’s essential features: his existence. God’s existence is inferred from the principle that posits that all that come into being necessarily has a cause. It follows that the type of knowledge of God that one has to firmly secure is that he is the creator of the world, or in other words, that he is its cause. Thus, God’s ontological primacy is chiefly interpreted in terms of causal primacy, and physical reality is to be understood as emerging from Him as its cause. Note in this regard the role of sense-perception in the apprehension of God’s causation. The observation of the bodily and visible world leads one to posit necessarily a purposive creator standing behind the teleological organization of the cosmos. In his treatises (including not only the two treatises On Providence but also the exegetical works), Philo enlists many ways by which the human intellect can infer by analogy the existence of the Father of the universe: from the observation of handiworks or works of art, or by the observation of his own soul. These are all different versions of the Argument from Design—an argument which has been crafted by Socrates (in Xenophon), but which has played a topical function among the various proofs devised by the Stoics for the existence of god. The identification and analysis of the passages in which Philo claims that by observing sensible reality, one can infer, by analogy, the existence of the creator, show not only that Philo integrated the Stoic proofs for the existence of the Gods in his theocentric philosophy—some crucial differences notwithstanding—but that he was moreover part of the debate revolving around Stoic theology. This debate, which has roots in the Hellenistic period and involves the Epicureans and the Academics, underwent a major reconfiguration and systematization in the Roman period. Thus, Philo is not merely an important witness, but also one of the actors in this debate.