Philosophical Investigations may well be the most important and influential philosophical work of modern times. First published in 1953, two years after the author's death, it is the last product of intensive work done by Wittgenstein during the period 1929 – 49, years during which he published nothing.
The work treats of the concept of meaning; of understanding; of propositions; of logic; of states of consciousness; and of many other topics which are, or have become, of central concern to philosophers.
Also available as a Blackwell Paperback: Wittgenstein's The Blue and Brown Books, Preliminary Studies for the 'Philosophical Investigations'.
The sketch of Wittgenstein on the cover is by Michael Drobil.