1. R.G. Collingwood, The idea of History (Oxford: Oxford University Press, 1963), pp. 9–10.
2. William H. Dray, Philosophy of History (Englewood Cliffs, N.J.: Prentice-Hall, Inc., 1964), p. 5.
3. Melvin De Fleur, Theories of Mass Communication (New York: David McKay Company, Inc., 1970), pp. 6–7. De Fleur is concerned solely with mass communication, but his basic questions can be applied to any communications system.