 The direct counter-model to Heraclitus’ explanation of natural changes was the atomism of Leucippus, Democritus, and Epicurus, who viewed matter as constituted by inert material atoms with permanent properties (e.g., weight) and took natural developments to be the macroscopic effects of atoms colliding and changing their spatial positions. Aristotle complained that the source of motion for these collisions remained unexplained and developed a philosophy of nature that includes a coherent account of the source of motion in natural occurrences, allowing also for explanations in terms of self-realizing and self-maintaining structural or formal factors. In Aristotle’s view an item in nature persists by the active exercise of a collection of capacities, a self-maintaining internal process organization (physis, or more generally morphē) that realizes a characteristic sort of functioning; by means of these characteristic types of functioning we sort entities into natural kinds. These kind-specific capacities must be realized within a matter or medium (hylē) that supports the relevant process organizations but that also harbors counteracting, disintegrative tendencies, due to the fact that the elemental components of matter (fire, water, earth, or air) actively strive towards their “natural place” further up or down. If Aristotle indeed took these active elemental tendencies as fundamental and allowed for elemental transformations as changes per se without an underlying substratum or prime matter (as argued in Gill 1989), he can be counted as a process philosopher. But Aristotle also supplied in his characterization of substance (ousia) those aspects that subsequent history selected to form the basic tenets of the paradigm of ‘substance metaphysics.’ In addition, substance-metaphysics could also draw on Aristotle’s classification of changes (kineseis) into generation, destruction, alteration, and locomotion, since this classification rests on a sophisticated doctrine of “potential” vs. “actual” features that are all attributed to a static subject of change. Aristotle’s distinction between kinesis (in this context: change, transition towards a goal) and energeia (in this context: activity), on the other hand, must again be counted as an important contribution to process thought. Aristotle’s distinction is formulated in relation to agentively initiated changes and activities, but the logical distinction arguably extends beyond actions to occurrences in general. Changes such as building a house or learning lead to an end (telos) that lies “beyond,” and are incomplete as long as the end is not reached—“someone who is building a house, has not built it” (Metaphysics Theta 6). In contrast, activities such as seeing or living well are ends in themselves and complete at any moment for as long as they last—“he who is seeing has seen.” Aristotle’s distinction between changes and activities is significant in its own right, but even more seminal for present-day process philosophy, since it is the first attempt to classify types of occurrences in terms of logical and linguistic features of associated expressions (see section 3 below).