: Pierre Bayle is the most forthright and systematic early modern proponent of the actual parts doctrine, the period's counterpart to the 'doctrine of arbitrary undetached parts' familiar from current analytic mereology and metaphysics. In this paper I introduce both the actual parts account of the internal structure of matter and the rival system of potential parts. I then identify Bayle as the leading advocate of the actual parts doctrine and examine his arguments for this account.