 Motivating this question was the traditional problem of the external world. If our knowledge of the external world comes through inferences to the best explanation, and if such inferences are always fallible, what guarantee do we have that our beliefs are true? Russell’s response to this question was partly metaphysical and partly epistemological. On the metaphysical side, Russell developed his famous theory of  logical atomism,  in which the world is said to consist of a complex of logical atoms (such as “little patches of colour”) and their properties and relations. (The theory was crucial for influencing  Wittgenstein’s theory  of the same name.) Together these atoms and their properties form the facts which, in turn, combine to form logically complex objects. What we normally take to be inferred entities (for example, enduring physical objects) are then understood as  logical constructions  formed from the immediately given entities of sensation, viz., “sensibilia.”