Wittgenstein, in his Tractatus Logico Philosophicus, a expressed the logical atomism philosophy.
It is the belief that analysis of the conditions necessary to give a sentence a definite meaning reveals that ordinary (molecular) sentences (or propositions) must be compounded from fundamental (atomic) units of meaning.
The elements of such atomic facts must refer directly to the basic entities whose relations make up states of affairs in the non linguistic world.
Russell and Logical Atomism
In Russell, although not in Wittgenstein, anyone understanding the sentence must be directly acquainted with these entities. Russell‘s versions thus gives rise to a reductionist programme, and may be said to be the precursor of logical positivism. For Wittgenstein the nature of these atoms was of no interest: they had simply to exist in order to make possible our actual understanding of everyday language.
This atomic philosophy was subsequently abandoned by both philosophers.