Philosophy of Epicurus of Samos (342-271 BC) and his followers, notably the Roman poet Titus Lucretius (c.99-c.55 BC).
They developed the atomism of Leucippus and Democritus, and like their contemporary rivals the Stoics they were materialists, though their atomism involved the existence of empty space, which the Stoics rejected.
Atoms fell downwards through space, but were subject to a random ‘swerve’ which served to explain both their agglomeration into objects and human free will (the latter application being subject to the same objection – that free acts are random – as the modern use of Heisenberg’s ‘uncertainty principle’ for that purpose).
The Epicureans had little interest in logic (unlike the Stoics) but their epistemology and ethics both gave primacy to sensations. In ethics these could lead to pleasures, which were ‘static’ (associated with states) or ‘kinetic’ (associated with processes), and were to be pursued; but the need to avoid pain led to heavy emphasis on ‘static’ pleasures of a calm and self-sufficient kind (though the value of friendship was stressed).
Epicureanism was thus very far from the modern sense of ‘self-indulgence’.
A A Long and D N Sedley, The Hellenistic Philosophers (1987); translations with commentary