David Furley begins his work The Greek Cosmologists with a concise presentation of the differences between two main approaches to natural philosophy in antiquity. One of these, the teleological tradition, best represented by Aristotle, understands form to be explanatorily irreducible and holds that teleological explanations cannot be omitted from a complete account of the natural world. Philosophers in this tradition consider matter to be continuous and to have no imperceptible microstructure; they regard qualitative change as fundamental and not reducible to rearrangement of smallest parts; and they think of the material cosmos as structured and finite in extent. The atomists, by contrast, take all change to be fully explained by the spatial rearrangement of these smallest parts, without reference to any purposes these changes might be thought to serve. They take matter to be composed of indivisible smallest parts moving in a void, treat macroscopic structures as explanatorily reducible to the properties of the smallest parts and regard the universe as infinite and unstructured.

It is important to notice that the contrast between two competing approaches is not presented as a logically exhaustive dichotomy: neither in the ancient nor the modern world are these the only possible explanatory options. Furley's point is not simply to segregate philosophical positions according to an exhaustive and exclusive dichotomy, but to note an interesting tendency of philosophical positions to cluster around certain key assumptions.

To resume: this argument for the existence of a ‘mechanical hypothesis’ in ancient Greek natural philosophy began with a conceptual examination of the meaning of the terms ‘mechanical’ or ‘mechanistic’. I distinguished the use of the term ‘mechanical’ as a systematic description of perceived features of a view from the idea that the thinkers in question were explicitly motivated by their own conception of mechanics in formulating a view. Some ancient Greek thinkers can be described as ‘mechanistic’ in the latter sense, inasmuch as they did appeal to the field of mechanics to understand the natural world. This history has received less attention than it deserves.

A tendency to describe the ancient atomists as ‘mechanistic’ seems to have deterred scholars from looking for the impact of ancient Greek mechanics on late antique philosophy. Historians of science, meanwhile, have mistakenly concluded from some remarks of Galileo's that, in antiquity, mechanics was systematically excluded from natural philosophy. In the other direction, a few histories of technology think that ‘mechanistic conceptions’ can be found as far back as Homer. I set aside some misconceptions before studying the impact of ancient Greek mechanics on natural philosophy.

Tracing the history of mechanics, I argue that there is little evidence of it – in any sense of the term – from before the fourth century bce. Branches of the field were clearly developing in the fourth century. Nonetheless, there is little definitive evidence of what the term mēchanikē meant to Aristotle.

One of Epicurus' central concerns is to show how human agency exists within a world whose ultimate constituents are simply extended and indivisible bits of matter (atoms) whizzing around in empty space (the void). A common way of putting this concern is to say that Epicurus wishes to defend free will against the threat of determinism. After all, the Epicurean poet Lucretius asserts that libera voluntas - often translated as 'free will' - is incompatible with causal determinism, and Epicurus (famously or infamously) posits an indeterministic atomic motion, the 'swerve', in order to defend our freedom against this threat. However, framing the issue this way risks anachronistically oversimplifying and distorting the Epicurean position. First of all, Epicurus defends at least three different sorts of freedom: “(a) Effective agency, our ability to act as we wish to in order to get what we desire. / (b) Self-formation of character, our ability to modify our desires, hopefully in a way that allows us to attain happiness. / (c) Moral responsibility, our ability to be justifiably subject to praise and blame for what we do.”

Although 'psychology' does not constitute a distinct sub-division of Epicurean theory, the term can be used to refer to a number of well-marked topics in their philosophical framework. These include (1) the bodily nature of the psyche, (2) the atomic composition of the psyche, and (3) links between psychological functions and the structure of the body. These topics fall, broadly, under 'physics' in Epicurean philosophy. However, the bodily and atomic nature of the psyche also has implications for Epicurean ethics, implications that can also be seen as part of their thinking about psychology. These implications include (4) the capacity of the psyche, in human beings, for the development of agency and responsibility, and (5) the mortal nature of the psyche. Also relevant is (6) Epicurean use of psychological language in ethical contexts, often without explicit reference to the bodily or atomic nature of the psyche, and (7) the psychological assumptions underlying the Epicurean therapy of beliefs and emotions. The focus here is on the first four topics, illustrated by reference to certain key sources, and on the issue of how to understand the theory of psyche-body relations implied in these ideas (Sections II-V). However, there is briefer comment on the latter three topics, indicating how the conception of psychology involved in those topics depends on, or is at least compatible with, Epicurean thinking about the psyche as bodily (Section VI).

Epicurean physics is fundamentally atomist. This means that it rests on two principal theses. The first is: 'All bodies are either indivisible small bodies or else are composed of indivisible small bodies.' The Greek adjective 'atomos' means 'indivisible, what cannot be divided'. When made into a noun, or when a noun is understood, usually it is translated as 'atom' or 'atoms'. Let us call this thesis the 'Atomist Thesis'. Epicurus sets it out in a canonical form at the beginning of his inquiry into physics at Letter to Herodotus 40: 'Amongst bodies, some are composites (sunkriseis); others are those from which the composites are made. These latter are indivisible (atoma) and unalterable ' Lucretius reprises the argument in the first book of the De rerum natura (DRN), calling the atoms 'principles' (principia) or 'the first elements of things' (primordia rerum). The 'composites' are rendered as concilia. It appears that Epicurus used this argument on various occasions, at least if we believe the scholion which accompanies this passage in the Letter to Herodotus: 'He also says this in the first book of On Nature and in books XIV and XV, as well as in the Great Epitome'.

Descriptions of Epicureanism in Rome often end with Lucretius in the first century BC. No innovations are expected of Epicureanism under the Principate and, in fact, anyone expecting to widen our knowledge of Epicureanism through the study of Imperial sources will often be disappointed, since Epicureans in this period mostly pronounce the familiar doctrines, while anti-Epicurean polemics, from pagan and Christian camps, are content to draw on the arsenal of well-rehearsed arguments, almost always aimed at Epicurus' materialism, his rejection of providence, his denial of the immortality of the soul and his hedonism. Certainly, Epicurus' teachings were not particularly favoured under the Principate. Throughout the first two centuries down to the time of Marcus Aurelius, Hellenistic philosophy, by comparison with resurgent Platonism and Christianity, was indeed favourably viewed; but even then Epicurus' teachings stayed in the background in comparison with Stoicism. His philosophy was eventually to lose all significant influence when, in Late Antiquity, Platonism and Christianity became dominant. One reason for the growing neglect might have been the alleged atheism of Epicurean doctrines.

A goal of Epicurean philosophy was the achievement of calm and freedom from anxiety. Epicurus believed that if people can be freed from fear - including fears relating to the actions of gods - they can then achieve ataraxia ('being undisturbed'). Epicurean cosmology and meteorology were motivated by the desire to alleviate fear of gods. While Epicurus recognized the existence of gods, he denied the possibility that they have any cosmic influence. He developed a strict materialist philosophy, designed to offer natural explanations of phenomena that were often seen as due to activities of gods. Questions about the origin and order of the world, its possible beginning and end, are potentially disturbing: violent natural phenomena, particularly thunder, lightning, hail and earthquakes, can be terrifying and destructive. If such phenomena are not due to gods, there is no reason to fear the gods' involvement in our world. Epicurean meteorology explained the meteōra (the phenomena of the sky, and earthquakes); cosmology focused on the nature of our local cosmos (kosmos), while acknowledging the existence of an infinite number of kosmoi (worlds). The Greek word kosmos carried a range of meanings; its use in natural philosophy was coloured by the worldview of the user.

“Philosophy, as long as a drop of blood shall pulse in its world-subduing and absolutely free heart, will never grow tired of answering its adversaries with the cry of Epicurus: 'The truly impious man is not he who denies the gods worshipped by the multitude, but he who affirms of the gods what the multitude believes about them'.” Karl Marx, Foreword to his 1841 Doctoral dissertation / “As you say of yourself, I TOO AM AN EPICUREAN. I consider the genuine (not the imputed) doctrines of Epicurus as containing everything rational in moral philosophy which Greece and Rome have left us.” Thomas Jefferson, Letter to William Short, 31 October 1819 / “In addition to removing all hope of help and favours from the gods, as we said, Epicurus blinds the part of our understanding that loves learning and the part of our practical reason that loves honour. He packs them tightly into a narrow vessel and removes any pure pleasure from body and soul. He degrades our nature, as if there were no greater good than the avoidance of evil.” Plutarch, Non posse 1107C