 Plutarch does not defend the Socratic-Academic epistemology only at the theoretical level, but also applies it practically. While discussing in On the Principle of Cold whether cold is a principle rather than a privation and whether earth is the primary cold element, he defends suspension of judgment as the right attitude to take on the matter (955C; see Babut 2007, 72–76 contra Boys-Stones 1997b). This is indicative of Plutarch's attitude to natural phenomena quite generally. He maintains that natural phenomena cannot be understood merely by means of investigating their natural causes. The discovery of the immediate, natural causes, Plutarch argues, is only the beginning of an investigation into the first and highest causes, which are intelligible (De primo frigido 948B-C; Donini 1986a, 210-211, Opsomer 1998, 215–6). In other words, a metaphysical explanation in terms of the Forms and god, the creator of the universe, must be sought (De def.  or. 435E-436A). This is what, for Plutarch, demarcates the philosopher from the mere natural scientist (physikos; De primo frigido 948B-C), a distinction further exploited by later Platonists (e.g. Atticus fr. 5 Des Places, Porphyry in Simplicius, In Physica 9.10–13; fr. 119 Smith). Plutarch is guided here by the dichotomy between natural and intelligible causes found in Phaedo 97B-99D and Timaeus 68E-69D (Opsomer 1998, 183). Explaining the physical world through an appeal to natural causes alone is insufficient, Plutarch argues, since such an explanation ignores the agent (god) and the end for which something happens in the world (De def. or. 435E). The fundamental ontological and epistemological distinction between the sensible and intelligible realms suggests to Plutarch an analogous distinction of corresponding levels of explanation (Donini 1986a, 212, Opsomer 1998, 217). Plutarch maintains that there are two levels of causality, physical and intelligible, and full understanding of natural phenomena requires the grasping of both. While in the case of natural phenomena suspension of judgment maintains an unfailing spirit of research, in the case of the divine realm, where human understanding is seriously limited, suspension of judgment, Plutarch suggests, is due also as a form of piety towards the divine (De sera 549E; Opsomer 1998, 178–179).