First Published in 1999. This is Volume XV of thirty-eight in the General Psychology series. Written in 1949, this text seeks to explain how we come to believe in our common-sense world, and why, in spite of all philosophical criticism, we cannot help still believing in it. The aim is to show how we progressively build up the various constituents of that belief, and how those constituents tend to support and reinforce one another in a single, well-consolidated structure.