It would be difficult to exaggerate our indebtedness to the center for Advanced Study in the Behavioral Sciences. The opportunity there provided for intensive thought and study without the interference of interruptions or the pressures Of deadlines is to be found in none of the other situations Open to the contemporary social scientist. These special circumstances enabled us to set a much higher goal for our work than otherwise would have been possible and to adopt a time perspective suited to its attainment. TO the staff Of the Center and their total devotion to the welfare and freedom Of the Fellows we Shall always be indebted. The stimulating input from our colleagues at the Center must also be mentioned prominently among its facilitative features. Kenneth Arrow, A. H. Halsey, William Lambert, Theodore Newcomb, Richard Savage, Harrison White, and John Whiting contributed in various and important ways to our work.