The Joint Philosophical Program of Russell and Wittgenstein (March–November 1912) and its Downfall

After finishing Principia Mathematica in 1910,
Russell concentrated his efforts in questions of
epistemology. In �Knowledge by Acquaintance and
Knowledge by Description� (1911), and The Problems of
Philosophy (written in 1911 but published in 1912), Ch. III,
he developed the epistemological implications of his logic
in detail. At that time, Russell accepted that whereas we
are acquainted with sense-data, we know physical objects
by description only. More precisely, we logically infer that
there are physical objects from the sense-data we are
acquainted with. In 1911 Russell believed that this
understanding is much more coherent than any competing
philosophy of matter, for example, from solipsism,
according to which there are no physical objects at all or
from naive realism, according to which we directly
apprehend material objects.